I'm having a lot of trouble visualizing what "Hadrumentum" corresponds to.
BTW, how many instances , in the course of gaming out how the intersections of evolving civilizations might shift important trade routes, create pressures on strategic borders that were of little consequence OTL, and so forth, have there been of either raising up obscure localities into major centers, causing you to have to rummage around rather misty and dubious sources to come up with local names and so on, or vice versa strangled OTL places of note by depriving them of their ancient causes for being the particular center they became OTL? The way I tend to assume things work there would be rather little of this happening; the names and cultural history of a site can change dramatically but the big centers presumably are rooted in centuries or thousands of years of holding an advantage of some kind that persists in importance, which in turn assumes that major sources of trade goods and major markets for them tend to remain in roughly the same place no matter who is doing the cultivating and other productive steps, who is doing the trading, and who the consuming. One might say for instance that the cities around Trier OTL might not exist had the Romans settled on some other boundary, but I'd say there were deep reasons they would push to the Rhine and not much beyond, and nerfing Rome badly enough sets up some other power in Gaul seeking and finding a boundary there, not farther south and east, and not far beyond. But I suppose eventually one finds a place whose history is dependent on something pretty capricious--it is indeed quite often that an old center will decline or vanish completely while a new one nearby is set up in its place. But sometimes that too seems inevitable in sequence; the OTL kingdom of Portugal could not preempt the role of Spain; Amalfi probably could not avoid being surpassed and cut off by Pisa, Genoa and Venice. Invading conquerors might have sustained Babylon instead of creating new cities in the near region, but perhaps one motive of these Classical and later invaders was that the old sites had become unsustainable despite heroic efforts to keep them so by the old regimes, between desalination and shifting river courses perhaps.
It is certainly exciting to see Mediterranean civilization taking deep root on the Atlantic shores of southern Gaul, reaching to Amorica and thence to Britain. I did wonder if the Carthagenians had considered striving to make a secured overland shortcut to communicate with these shores overland from the Mediterranean. Indeed it seems the effort to bypass Emporium might have had that in mind too, as a secondary task however, and that it came to grief. So had Carthage's supremacy been more sustainable, maybe there would have been a drive to do that?
But others point out the Gaulish colonies are not really a Carthagenian effort at all but a tertiary wave coming out of Iberian territories the Carthegianians regarded as under their sway but which slipped out during the general crises putting a check on a generic Phoenician empire; even if a concerted drive by Carthage to get direct communications were undertaken it would be uphill all the way, with the locals on the Atlantic little inclined to cooperate. Of course in this era sea communications are so much more economical that even a short isthmus overland is more costly to cross than a very long circumnavigation of whatever the isthmus connects to; a "direct" connection would be of little economic value (and what little it might have would be in competition with Massalia) but at best a matter of saving time--which is why I have been using the word "communication" rather than "commerce." If Carthage or any other Med super-city could establish a more or less Roman like domination then such things become important; if there is a patchwork of regional states instead no one particularly cares.
Before replying, I'd just like to say I sincerely and wholeheartedly look forward to your responses to the updates, it's one of my favourite things about writing the timeline. I always get excited about people replying but yours in particular have a depth and consideration put into them that I relish, and I feel genuinely complimented that what I'm writing provokes that level of thought.
For your first issue that you're having some trouble with,
wikipedia to the rescue! It was to the south of Carthage and potentially older, the 'second city of the Carthaginian Empire' status was really mostly claimed by Utica but all of the other old colonies of Africa had similar pretensions from what we can tell, and I see no reason why the Old Cities club of Africa would have been content for just one of their number to replace Carthage when that level of hegemony was probably resented in the first place.
As for raising up obscure centres vs depreciating older ones, there has been some of the first but not very much of the second. Most important cities that have shown up in the timeline were genuinely important in some fashion in OTL, but the exact level of their significance has varied. I absolutely agree that there's some underlying factors that tend to promote certain cities growing for certain reasons and having a reason to become more powerful. But politics and human willpower do have a tendency to ignore purely pragmatic concerns, not to mention introduce a certain wild card factor in the promotion of certain cities or the depreciation of others in OTL. Argos and Syracuse both enriched themselves by stripping neighbouring poleis of their populations by force, Alexander chose to keep Babylon as capital whereas Seleukos committed to an entirely fresh urban foundation in Mesopotamia, Thebes and the rest of Boiotia were seized after the Persian Wars as punishment for supporting the Persians, Sybaris was destroyed by a coalition of neighbouring poleis, Rhodes was awarded huge swathes of land in Anatolia because of its importance to feuding Hellenistic states. These and many, many other such incidents are hugely subject to the whims of particular people and the outcomes of particular wars.
Alexander did not build new cities but he did refound a number of important fortifications, the presence of Macedonian settlers and garrisons making them more important to his successors than they might otherwise been, the choice of which sites were refounded hugely depending on his specific campaigns within the Achaemenids' borders, and then without the Seleukids would we have had a specific situation in which a Greek ruling class did not rule its own homeland and have to found immense numbers of new cities across its Empire with imported Greek colonists? The list goes on and on. And there are other ways in which the purely practical growth and emergence of cities can be affected by more organic events; whether the place is self-ruled or subject to a foreign power, if it's subject to a foreign power whether it's of particular significance to that power, does that power consider this location rebellious or loyal, if it's self-ruled is it part of a larger state, is it at the frontier of a state or the heart of it, has the site got a wider cultural significance beyond its vital statistics, is there an organised system of labour and government capable of clearing marshes or dredging rivers or building irrigation canals. History and the whims of people is the difference between Byzantion the trading city, mostly known for being conquered by anyone securing the Bosporos or Black Sea grain routes, and Konstantinoplis the Queen of Cities. Organic affairs is why there was a Spain with those possessions and borders at that time vs Portugal with its, or why Genoa, Pisa, and Venice were free agents able to act so concertedly and independently against Amalfi.
You definitely recognise the capriciousness of history but I think you underestimate it. This is certainly a timeline that revels in a reasoned capriciousness, after all the very PoD in question is based on that principle, and this is a period in which the very terms of continental Europe's urbanisation were being set. I've not been raising up centres that had no significance OTL, but I am also trading on a slightly different notion of obscurity to a lot of people; in my academic life I moved outside of the usual suspects in Classical history a long time ago, and discovered that famous doesn't always equal important. This is a timeline where reasoned capriciousness is fighting tooth and nail against practical, set in stone factors all the time, which is why Carthage will remain a major city even after being deposed as head of its own Empire, and why it's Burdigala/OTL Bordeaux that becomes the chief colony of Tartessian Aquitania, and why Gergovia will be one of the biggest cities in Southern Gaul, and why Capua remains a city of growing importance throughout these centuries. Rest assured, where a city unfamiliar in the hitlist of OTL ancient cities turns up, it's either a genuinely important but neglected one from OTL or one which has seen an improvement in its fortunes in TTL or it's a site that has distinct advantages. Sparta might be gone but there will be a city in Lakedaimonia that will replace it as a local centre of government and power.
My instinct about Carthage is that they would have expanded into the territory of the Elisyces in TTL if they had kept going, establishing a colony at Narbo, and then branched overland into the Garonne valley and then the Gironde Estuary, though I doubt they'd have actually managed to take Burdigala in that timeline for some while and would instead have had substantial overland outposts on the road out of Narbo more like fortified emporia or staging posts. Then again, had Carthage not come to grief, I'm not sure the entire Tartish Atlantic project would have come to fruition, since that was partially a response to Gadir's strangehold on the Pillars of Hercules, and any colonisation in Southern Gaul would have remained Med-centric. As you say, it would only have been a really 'sensible' project as part of an effort to really go for the imperial dream in that part of the world, where they were already committed to securing and pacifying the entire region so hang the expense and the irritable tribes. For Tartish, however, the Atlantic is their focus, and their core in Iberia, they can afford and indeed need to secure some of the overland routes from the Atlantic coast to the Mediterranean so they don't keep getting utterly fleeced by the Gadirians, and can also connect up to the Etruscan/Greek favoured routes into Gaul. This wasn't my intent when making it but it rather reminds me of how the Sogdians had their caravans going from Samarkhand all the way to Chang'an and the Indians linked up to that route through Himalayan passes rather than going round the west.