Every AMD product I ever had had the same problem. It's one of those "you get what you pay for" issues, the planes are still a FANTASTIC value. The whole thing is easier if you just adress it right out of the box. Tear out those mounts and put in new ones. Any futher engineering you want to do to spread the loads on those mounts is cool, too, as the basic design is not particularly brilliant.
But the lite ply thing...even doubled up, that stuff won't really hold screws, it's too soft. Even IF the mounts hold, the screws will come out. Unscrewing the screws again and hardening the threads with CA will give you a few more flights, but it's just a temporary solution. Don't confuse "lite ply" with real plywood. I cannot tell by looking at the pic, but EVERY other AMD plane I have ever seen(except for this Reaper I have right now) had the same material in the mounts.