If I go with Zar on the next
: polyester skin, will it fill these holes OK? Should I be sewing
: the polyester on looser? Any guidelines on how tight to snug
: this up while sewing? I noticed there are a number if different
: options with the ZAR polyurethane. Is there a specific type I
: should use?

Use half dried, thickened, varnish to fill most sewing holes - just like with paint.
ZAR exterior oil based polyurethane varnish is what I prefer to use. Works fine with polyester, and you can tint it with a bit of oil based paint.
As for sewing, you don't want to pull it as tight as nylon, though pretty taut is a good idea. Your heat source should do the brunt of the shrinking.

Good luck!