Re: stringers
Cut the past the rotted section section ~ 6 inches. Grind down the fiberglass to expose the wood and stick a heat lamp on it for a couple days or until it is bone dry. Cut a scarf out of the old stringer and join it with the new marine ply with some thickened resin. Then like you said put serval layers of mat and roven or cloth. It is easier if you get some stitchmat which has roven or non-roven with glass mat stitched into it.
