I think this will help you on the repair you are working on. When ply wood that is in place needs to be scarfed it can be done with a lap scarf. Not as neat and clean as a tapered scarf but it works. Basically you need to cut the mating surfaces in half to fit into each other. This can be done with a hand held router and a chisel. Set your router at 1/8" when working on a 1/4"panel and cut a checker board pattern. Use a chisel to clean it up.
Do this to both pieces. Sand, dry fit and invent a way to clamp it all together.
Now just wet the mating surfaces with straight epoxy, add a frosting of epoxy thickened with microfibers and clamp. Be sure to take the time to invent your clamping system first. Every project will call for a new clamping scheme.