Why not just use the tools that the rest of us use to generate Mobi files - Book Designer or the Mobi Desktop Reader - instead of beating yourself up over something you have no control over?
Yes, Mobi files are Palm Resource (PRC) files; the "BOOKMOBI" string you found is what identifies the specific data contained in the file. "TEXTREAD" is an alternate that you'll find in other Mobi files.
Mobi is pretty much a
de facto eBook industry standard - it's certainly available on far more devices than any other format, and has more books available than any other format. There's no point of having hysterics over the fact that it's a rather elderly file format; it won't change and you'll give yourself a heart attack
.