A potential to consider: regardless of the outcome of this war, whichever Sultan of the Porte signed off on this treaty is going to become incredibly unpopular for it among patriotic and nationalist circles. Then factor in the Great Eurasian War, and the fact that even if the Ottomans avoid being stomped by the Russians (and perhaps Persians), this will have been a devastating and extremely ill-advised conflict. Through both of these, I see Ottoman faith in the U.K. being very strongly shaken - the Brits pressured the Porte into giving into them and starting this war with Russia, then backed Greece's demands for massive concessions of irredenta in exchange for peace.
Could this be an opening for a palace coup and the installation of a new sultan? If that were the case, then who is to say that the Porte would continue to uphold the Treaty of Constantinople? The author did mention that Thessaloniki would be Greek sooner than one would expect, and a revanchist conflict before the Porte was recovered from the GEW would be a good opportunity for this to happen.
Abdul Mejid I died in 1861 anyway. That said Greece here got from the Ottomans what it had been promised in OTL 1878 Russo-Turkish war for its neutrality plus the Dodecanese, while being n a relatively much stronger position. What happens post that? It's not difficult to bet on Bosnian and Bulgarian revolts no later than the OTL 1876 and perhaps earlier. And if these are treated by the Ottomans in their customary fashion it means anything from a Balkan war, to another Russian war, only with the British public against their government joining on the Ottoman side. Then you have such complcations like Hungary, an unknown factor I'd say, Egypt more likely than not eying all of Syria and more while allied to France, the Italian confederation, a likely Greek ally TTL and Armenian nationalism growing far faster than OTL...
Side note Greece and Serbia officially allied in 1866 in OTL even if it went to nowhere. Of Serbia we have heard nothing about so far, besides keeping out of the fray but if they are to remain neutral in the longer term they have to be given something... like full independence and Turkish troops being removed from Serbia. In turn if full independence is achieved under Alexander he's likely not overthrown and we have the Karageorges as the Serb dynasty without interruptions...