Honestly, i think some stuff that donovan pulled off is right up in the NG tier enemy, Zombie Raidou, Omega, Alpha and Phase 4 are probably great-fiend tier...
I don't think so, if ryu fights them he'll defeat in in less than a few moments. I honestly think he needs to back off and leave it to Kasumi, it's her game after all
I can tolerate him being in DOA, but not stealing Kasumi's thunder, that's what ninja Gaiden is for