I'm not sure why you want to install fiberglass over plywood for a deck surface - exterior at that . . . The plywood will delaminate over time, and the fiberglass has some problems as a walking surface (I'll admit that I don't know much about that). So, if that what you want to do, I can think of one solution to improve the look of your deck. Wrap the perimeter with a continuous, level band board. You can use PT wood, cover it with fiberglass panels if you like, whatever. If it is level and deep enough to cover the sloping side of your deck, a perimeter band will improve the look of your deck's edge condition. I recommend sealing it at the seams if you plan to butt it against the fibergalss walking surface - wrapping the fiberglass membrane up the sides, over the top and down the front face would be ideal. Or, hold it away from the side of the deck (structural beams and walking surface) about 1/2" or so water can potentially drain over all of the edges - this scenario eliminates the concern you may have about water freezing in the cracks/joints.
Good luck. So, why have you chosen this method and materials anyway?
1/4" per foot is a lot of slop for a smooth surface. I am pretty sure 1/8" would be plenty to allow it to drain well. You may even "feel" the 1/4" when standing on it.