How to recover corrupted Electrum wallet.dat
If the file is corrupt, it will require manual editing to be able to open it with electrum.
Depending on how corrupt the file is, it might be possible to recover and and it also might not.
Say, for example, a single bit is flipped. Then a brute-force approach might work where every bit is flipped in the file and then check which result can be opened and decrypted with electrum..
Check with electrum developers who are most familiar with how a non-corrupt file should look.
If I were in this situation myself, what I would also do is generate a few new wallet.dat files using the same electrum version you used in 2015, open them in hex viewers / text editors next to each other, and try to see the patterns. Compare to your corrupt file.
Good luck.