It won't. They survived 1395-1402 because Timur came to Anatolia. There is nothing that stops the Ottomans. Other than divine intervention of course...
If it survived the Ottomans would return soon.
True, broadly the only thing that could save it is the Ottoman empire tearing itself apart and that is not happening. At worst if Mehmed fails he's strangled or at least replaced for a second time as Sultan. The Turks then go away, build many more guns and come back in a year or so and that's that.
The walls can't fight gunpowder and the population is below 50,000 at this point so manning all 14 miles is a dicey prospect anyway. Wall technology to deal with siege guns is decades away and the Byzantines have no money or resources so replacing the walls before a second attack isn't happening.
The Serb's and Hungarian's may fight but both are already quite weak (their own final declines are imminent) and getting involved in round two is likely to annihilate their armies and speed up Ottoman domination. Also the daughter of the Serbian King is the former sultan's widow and Mehmed's mother figure and she will doubtless try to influence her father to stay out of round two as well.
Latin involvement beyond Mercs is likely fatal to Byzantine chances since as others mentioned the schism is now so bad that Islsmic domination is preferred to accepting papal superiority. Especially as the Ottomans had been quite canny and pretty much allowed their subjects to keep their faiths with payment of a tax. In contrast letting Rome back in is heresy and would stir ugly memories of 1204.
In the end all that can be done is to buy a broken, crippled city a few more years before the Ottomans come back and finish the job. At this point better to put the city out of its misery and start towards a new golden age under the turk.