They pretty much achieved the same results in OTL, albeit they reclaimed this territory in 1829 as opposed to 1828 in TTL. The OTL Morea Expedition took nearly an entire year to transfer control of the castles from the Egyptians and Ottomans to the Greeks, the Ottomans were especially resistant to surrendering their castles and actively fought against the French. It helps that in TTL, the Egyptians only ever managed to secure the western coast with any effectiveness, and the sudden exit of the Egyptians from the war prevents the Ottomans from dispatching forces to the region to take possession of their castles.And so the greeks on the mainland arrive at otl borders more or less victorious. Now the true battle begins, which will decide if most of the islands and Crete make it into this new greek state..
Across the Gulf, the Greeks are also a lot better off, as they are starting much further north than they were in OTL with Athens, Amphissa, Nafpaktos and much of the Morea and Central Greece still under Greek control at the end of 1827 instead of being under Ottoman control. Much of this has to do with Odysseus Androutsos still being alive and still being loyal to the Greeks, as a result they are much better in Eastern Greece, with the front near Lamia as opposed to Athens.
Another important thing to mention is the lack of Resid Pasha, the Ottoman Serasker and Grand Vizier who died during the Third Siege of Missolonghi ITTL. He was arguably the most competent commander for the Ottomans in the war, outside of the Egyptian commanders, which makes his death especially detrimental to the Ottomans in the long run. Without him, the Ottomans bungled the post Missolonghi campaign in Western Greece. So instead of besieging the Greeks at Athens in the Fall of 1826 and Spring of 1827, the Ottomans were defeated and subsequently besieged themselves at Missolonghi by the Greeks.
Regarding the islands, some of them will be included in the independent Greek state and some others won't it will most likely depend on what the situation on the ground is and how generous the Great Powers are in bolstering Greece territorially. While they are generally in favor of the Greeks, I can't see them getting everything they want territorially and making them independent as well.
Economically it will help quite a lot as more people and more territory means more taxes to collect and more trade that can take place. Crete was also a very important trade post for ships traveling to and from the Black Sea. Even today, Crete is one of the wealthier provinces in Greece due in large part to its strong agriculture industry on the island, although this has diminished in recent years with tourism becoming a greater part of the Cretan, and Greek economies.How would a territorially larger Greece postwar impact its future economically and politically? Crete could be difficult because of the Cretan Muslims (but Greek speakers) who were a large part of the island’s population at this point. Do they flee the island as OTL from sectarian violence?
Politically it would also help as post independence, there were still more Greeks under Ottoman rule than under Greek rule. This was incredibly humiliating politically and it delegitimized the Greek government in many ways. Crete also engaged in six recorded revolts against the Ottomans after the war for independence in 1841, 1858, 1866, 1889, 1895, and 1897. The Greeks on numerous occasions attempted to support their kinsmen on Crete, but were impeded either by the Powers, the Ottomans, or their own inability and fractiousness. Having Crete under Greek control eliminates a lot of these problems. Oddly enough, having Crete be a part of an independent Greece could possibly benefit the Ottomans to some degree as well.
Roughly speaking, the OTL population of Greece post war was about 700,000 in the post war census in 1834. I don't have an exact number on the population for Crete after the war, as the 1831 Ottoman Census took place when the Ottomans were fighting against the Egyptians who just so happened to have control of Crete at the time. As a result, the best I can do is estimate its population using the 1872 census which lists Crete's population at 120,000 Christians and 90,000 Muslims for a total of 210,000. Interestingly, this census was taken right after the Great Cretan Revolt of 1866 to 1869 so it could be relatively similar to the population of a post Greek War of Independence Cretan population, although one can't be certain.
The Muslim population of Crete post war will most likely flee the independent Greek state after the war as was the case with the Muslims in the Peloponnese and Central Greece in OTL, although a minority will certainly remain it won't be very large. The Christian population would also be somewhat lower than the 120,000 given in the 1872 census as they suffered from years of Ottoman and Egyptian occupation and deliberate acts of destruction. If I had to make a guess, the islands population, post war would be around 100,000 if it is included in an independent Greece.
I will also say that the land border will be a bit further north than it was in OTL.