I have to agree with most people's consensus here--a lot depends on the technical limits of effectiveness of Tabun even weaponized in the most brilliant fashion possible. Noting that Tabun is "persistent" I have to wonder, how persistent? I presume it must break down sooner or later, perhaps with some intermediate breakdown products being themselves harmful though it is hard to imagine as harmful as the Tabun! (Thinking of the analogy of hypergolic rocket fuels here--hydrazine and nitric acid, and the tweaked variants of each to improve their respective qualities as fuel and oxidant, are themselves incredibly toxic, but so apparently are products of their reactions. But neither are designed to be chemical death agents). At some point the breakdown products just merge into the general background noise of chemistry ordinary biology has to deal with. So what is the time scale? Years? Decades, centuries? Is it an exponential decay, or perhaps follows some other kind of curve with a definite point of total extinction? Can its breakdown be accelerated and what conditions retard it?
Anyway, there will be theoretical limits. Zeppelins can in principle, if they can learn to deal with the unexpected high altitude winds, and devise a suitably reliable navigation system, drop nerve gas bombs on London--they certainly cannot aim at a precise target. So it is a question of, how many people can a tonne of Tabun realistically kill, bearing in mind persistent effects--so that if one just dropped a big drum of it on some random point in London and it bursts on impact, a lot of people in the immediate vicinity will die, but these humans will absorb only a small fraction of the vapor, and much of it might not evaporate at all, persisting in droplets and slicks and seeping into the topsoil and drains etc. So some of it pools in the vicinity of the drop zone; at first anyone entering this zone would be pretty sure to pick up a fatal dose and die on the spot, but over time the pools that are most volatile and likely for a person or animal or bird to touch will be depleted, washed away by rain or just used up; little stocks of it would be sequestered here or there making inhabitation chancy, and other portions washed down the drains to cause death all downstream.
By 1918, big airplanes like the Gotha bomber and its Allied counterparts were eclipsing Zeppelin bombers, though they would still have some of the same problems. Anti-aircraft defense was improving as well, but I do think that given suitable bombs, which might be more efficient than just dropping a drum of the stuff as above (some means of aerosolizing it by large amounts, using many smaller bombs instead of a few great big ones, etc) German bombers can do serious damage behind the lines. But I believe other threads have tried to do the math and the upper limit of mass death would be tightly bound well below nuclear levels of destruction.
We damn well know, people do not surrender to terror bombing, no matter how terrible. If the bomb does not level the entire city and kill everyone in it at one strike, the survivors rebuild and continue to fight. They have no option to surrender, on a piecemeal basis, and any demoralizing effect making a negotiated peace seem more necessary is offset by anger and vengefulness.
In 1918, the German General Staff might be "forgiven," for not having data on how people react to terror bombing, if not for the obvious moral crime involved. It certainly seemed reasonable to many people that terror bombing would indeed cause the attacked party to put up the truce flag and start talking terms.
Arguably, Tabun is just the same as mustard gas or any other terror weapon of the Great War. You seem more interested in considering its use on the front lines. But weren't the Allied western front trench troops already equipped with pretty good anti-gas protective gear? Tabun might penetrate it--i high enough doses that is. Very light doses would be stopped by the same gear effective against mustard gas. It might be that as a persistent agent, Tabun would be carried into supposedly safe zones when the troops strip off the gear, and everyone dies there instead of in the trenches, but again I believe mustard gas had a similar risk of being smuggled into shelters, and procedures existed. So Tabun shelling sufficient to make a big difference to Allied troops would also be so intense as to kill all the civilians--but I suppose by 1918, most of these are either evacuated or have anti-gas gear and procedures of their own.
So, Michel, I respect you as a humane person and as an engineering mind who can do the math. You tell us, mathematically speaking, given that the Entente has had some years by now to adapt to mustard gas and there are gear, procedures and shelters to minimize that risk, what levels of shell production, agent production, shelling and bombing operations must the Germans undertake to make a major numerical dent in the Allied forces that survived the named offensives OTL? Won't we find that the magnitude of Tabun shells necessary to say kill half the survivors of the OTL last ditch operations is going to be so large that the CPs just plain cannot afford it, perhaps by orders of magnitude?
The most realistic outcome then is that it is just more horror of the Great War of OTL, and the same powers will "win" it, by the same means, and having devised a poison gas even worse than mustard gas will just be yet another item on the "War Guilt" bill the French tried to stick on Germany. The additional cost of life, both promptly during the war and postwar as various zones need to be evacuated and kept clear of human use, with marginal cases of death anyway happening years later, and considerable opportunity cost imposed on the gassed zones, is just another layer of general cost. Maybe France will demand and get annexation not just of the Saar but all the Rhineland, on the theory that the swathes of land ruined by Tabun shelling must be compensated somehow, and the German population will be rousted out--maybe France must share this annexation with Belgium, which would also see a lot territory ruined.
I can't really credit the idea that even France, let alone Britain and the USA, will come to terms. The French might if they were alone, but then again France alone could hardly have lasted until 1918 anyway, eh? The Americans and British have no reason to roll over; their people at risk (barring the occasional Zeppelin or Gotha air raid) are trained, equipped front line soldiers, who now have additional motives both to make best use of their protective gear--and make the Fritzes pay.
Nor can I believe Germany can produce and deploy enough Tabun to create a death zone barrier rendering the German heartland invulnerable. After all, troops in suitable protective gear can cross land drenched in Tabun; any German defenders would be under the same handicap. But the main thing is, how much tonnage of Tabun can Germany produce, and still keep up in maintaining other munitions while keeping the CP subjects properly fed and so forth?
I think if we want to pursue this grisly line, we do better to not suppose the German commanders will recoil in horror at Tabun's potential and react as though it were a WMD, and instead just immediately start substituting it in for mustard gas piecemeal. Perhaps Saddam's cocktail of mustard/Tabun becomes the standard issue gas shell?
I have to wonder how long it would take the Allies to reverse engineer it somehow, or failing that, for Entente intelligence to hit upon the secret and smuggle it out, so both sides are using Tabun before the war finally ends.
I don't believe any kind of gas will change the basic and overall game all that much.
