So was the previous owner actively sailing it before you bought it? IF yes consider how much you actually HAVE to do vs WANT to do before you can get out there.
Here’s a link on sandwich. The logic is it yields a stiff, strong structure, lots lighter and perhaps even stronger than one single layer of FRP.
This works as long as all three layers are intact and stuck together. Your problem is that the inside layer has lost its integrity so the whole thing is now weakened. I would closely examine the top and bottom layers of FRP to determine if they are cracked etc. IF both are OK then decide if you care about cosmetics - that drives repair from the top or bottom layers of the sandwich. To me it’s a lot “easier” to tackle this from the top where I’m not laying on my back in an enclosed space breathing nasty fumes and having hot chemicals drip on me
As we say here - your boat your choice.
en.m.wikipedia.org