If you can read the text properly at $100 then it's probably not interleaved.
(if it is interleaved, you'll only see every other letter, like "SEGA MEGA DRIVE" will become "SG EA RV" or something like that.)
I don't think that interleaving is even a format you could hack with unless you were completely nuts.
(unlike the SNES, where I did once hack an interleaved ROM before even knowing it was such.)
There should not be a 512-byte header. The "SEGA Header" stuff is part of the actual ROM data that Sega required developers to put in (and at least part of it was mandatory and required to run games on most actual consoles. Trivia: the western release of Populous doesn't work on most consoles because EA failed to include a Sega header on the game, despite that the game was released a couple years in and so they should've known. They were aware enough at least to put a warning on the box.)