Other than the pitching regulations, what are the differences between fast pitch and modified pitch? I'm scheduled to do a men's MP next week, and there are a few women's MP leagues I could be doing.

One specific question is regarding HBP. I know there is no HBP in slow pitch, and the rulebook says fastpitch only in 8.1.F

:D 8.1.F is also where it states "the hands are not to be considered part of the bat". Are the hands part of the bat in MP and SP?

All fastpitch rules apply to modified, with the ONLY exception to replace Rule 6 (Pitching Regulations) in its entirety with the Modified Pitch version of that Rule. That statement does include ALL rules which state "Fastpitch Only".

The hands are part of the body in all versions of softball (and anatomy, I believe). The point of excluding slow pitch is NOT to state that hands become part of the bat, it is to state that, in slow pitch, the batter does not become a batter-runner when hit by a pitch, as 8-1 (everything) states.