John,
Sorry for the slow response, was out of pocket until yesterday evening...
First, keep in mind that epoxy has a higher shrinkage rate than polyester or vinylester, plus it takes up to a year for the shrinkage to finally reach a point that it doesn't ghost, (print through), back under the paint.
This is really only an issue when doing high visibility work like you'll be doing...
1708 or 1208 biaxle is a good choice for the larger areas but needs to be buried beneath a couple of layers of chopped strand mat to keep the weave from ghosting back.
Mat can be used with epoxy but it won't completely wet out because the binder that hold the strands together is formulated to dissolve in contact with styrene. (See:
West System Epoxy Article on mat)
Make all of your epoxy modifications to stand above the finished surface by 1/8" and allow at least a couple of weeks for the epoxy to shrink down before grinding.
Your initial grinding should bring the modification down to about 1/32" below the finished surface to allow for fairing. The best fairing compound is Awlfair, (also an epoxy) has a very low shrinkage rate, sands easily and further prevents ghosting of the substrate.
Prep for the Awlfair is a sanding with 220 paper and a couple of layers of 545 Awlgrip epoxy primer, sanded with 320 paper. The longer that you can put off fairing the less ghosting you have to deal with. Final fairing shoud be done about a week before top coat painting.
Backing large holes with 1/4" plywood is fine, but try to make the backing larger than the hole so that you won't have ghosting issues from that perspective.