Hi Penta! Glad to hear from you.
You should know that the level header bytes are 99% known now, only one remains and that's dependent on the disassembly to work out what's happening with it -- it's used for flags during runtime, so has no known relevant value in a level header.
Don't worry about looking for bugs with MaSS1VE yet. I know just what needs doing and it's too early for bug hunting yet as it's waiting on further disassembly work. :)
If you've been looking at the sound driver, ValleyBell has done a full disassembly which I've put into my disassembly of the game: https://github.com/Kroc/Sonic1-Z80-ASM though I haven't integrated all of his comments on the sound driver yet, but if you're interested I could mail you his work. I've made immense progress with the game, large portions of the ROM are correctly labelled and commented now. If you'd like to help more, please refer to this as it's the most complete source of the game available.