Has it ever been shown to regrow limbs/molt?
If it doesn't go into specifics its rediclous to assume immunity, so it shouldn't have immunity to poison, possesion or disease. Parasites don't count as possesion.
They should at best get a possibly resistant to disease as theirs no garantee the disease could even transfer between them and the roaches.
What gives them poison resistance?
How long was it around? How long did it survive?
This is a giant mutant insect you cant really assume it shares the abilities of common insects...