You need to decide on how long you want it last and how much you want to spend.
I have no information on the plastic ply board, but if it's polyethylene then nothing will stick to it and none of the PE, PVC or other composite boards have enough strength to be of much help in a rebuild. The only thing you get is a shape to glass around.
You'll need to grind the glass no matter what method you use to do the repairs, there are no options on this, so you need to plan on it.
If you want a low cost, simple way to do it, just take out all the old wood leaving the glass in place, then glass over the upright portion that's left.
Do not poor resin in anywhere to fill up voids, it has no strength and will create problems, leave the old glass of the stringer hollow. drill a few holes in the lower area's of the stringer so water can drain through.
In part of this thread you said you wanted it to last a long time, which was the reason for the plastic ply board, in another area you said you wanted it to at least last through the next season. Anything will get you through the next season before rotting, so it needs to be one or the other.
Most people that do these repairs want the boat to last forever after the repair, the problem is they rarely keep the boat very long. So the high cost of the repair is to the benifit of the next owner, but they seldom pay enough for the boat to even come close to recovering the cost of materials, let alone the labor.
