Once the resin is cured, it won't emit any more fumes. If you're in a closed room, they will linger past that point, though. How long the curing takes, depends on the product and the amount of hardener you used. Check your resin's datasheet/package.
The fiberglass-resin-compound can break of course, but only at the same time and that would basically destroy the armour. Just breaking the glass and keeping the resin intact is impossible.
I'd apply one layer, maybe a second at potential weak spots and then just let it cure and check, whether it's hard enough. If not, you can still add more layers.