Do not have time at the moment to help you with all of the above, but this bit :
Also, since I've already epoxied over this area, could I put on another thicker coat, maybe with milled glass filler or something and see how it holds? I wouldn't want to get into an unsafe situation on the water, though.
Take the epoxy off again, all you are doing is sealing in water into your foam/mat.
I assume is the foam you should be 'below' the gelcoat and epoxy/mat layers
Other stuff, for the release mat, I assume we are talking about something like grease proof paper or such like which ill allow you to make a female nold.
As for the glass matt/micro ballons, get it from the same place you got the epoxy from....It will (prolly) come in 1m (3 foot) wide rolls that is sold my the m or yard and what is usually called glass mat 'tape' which is a bout 3 inches wide...
If you are really unsure, post aback here and I will go into the garage and take piccies for you :
Glass Mat
Glass Tape
Micro Ballons
Chopped strand
etc.
Don't have any foam at home, but it is exactly what is says it is, just fairly dense and will sound hard (ish) when tapped (but you wont be exposing this much) - It is white (usually)
