Well, obviously pictures aren't as handy as seeing the boat directly, but it looks like maybe some of the crazing isn't crazing so much as scratch marks. Maybe you used an agressive-grit sandpaper? Or a slightly beat-up scraper? I've never cleaned bottom paint - especially through a layer or two of barrier coat - and not scraped up the bottom a fair bit. I'm sure I'm not saying anything you don't already know when I suggest that crazing usually occurs around stress points - fittings, chines, 'grounding surfaces', etc. But it looks like you've got marks all over the place, which suggests that not all of it is crazing.
So if a some or most of those scratch marks aren't crazing, then it's not such a mysterious situation.. you simply have a quasi-pockmarked hull that's been filled with all sorts of things over it's life. Different coloured things.
If delamination is your concern, there are some pretty sophisticated tools that can measure such things. But from what I see, there's nothing to suggest any significant issues. In my experience, if the gelcoat is compromised, it's not difficult to peel away the damaged areas with a fingernail. I don't see anything like that going on here.
Additionally, since you sanded down to the fiberglass in some areas (I assume those areas are/were areas where there was some questionable 'stuff' going on) and didn't see any opaqueness from non-wetted glass, I hesitate to get excited.
I would take another look around, see if there are any obvious damage spots. If not, I'd simply sand, put on a healthy barrier coat, a good thick (extra coat or two) of ablative, and smile about the fine, fine hull you've got.
... or I'm wrong.
Living aboard, currently in the Chesapeake
O'Day 37, still new to us