It's impossible to tell what is going on from the PDF photo in terms of layering. At this point, the only one who could tell you is the guy that sanded it down as he was observing the layers being revealed as he was working.
Looking at the photos in your original post, it looks like grey barrier over gelcoat
with blue/green anti-fouling on top of that. Looks like the yard in Mexico did not fill all the blisters after opening them but simply painted over with bottom paint. It also looks like the WA yard's aggressive sanding
went right through the barrier to the gel. But that's just a guesstimate based on what's visible and to Minaret's point, it's a crap shoot trying to figure out what's going on without an in-person inspection
How to proceed is a function of how much you want to spend vs. the longevity/soundness of the solution. At one end of the spectrum you could take the whole hull
down to the gel, fix the blisters properly, barrier coat the whole thing and repaint. At the other end you could just clean out the blisters, fill, barrier coat them, then repaint. If it were my boat and I hauled it every year for the winter, I would probably go with the simplest solution and then if I had a problem develop later on, contemplate a full-on hull refurbishing.
As Minaret said, it all comes down to adhesion. It also comes down to water
penetration if the barrier coat is gone in certain areas, but that's not the end of the world depending on how you use your boat.