Welcome to the new and improved Proflex / K2 Tech Forum!
For your first question the 755 and 855 are exactly the same bike the only differece was componets. So yes the risse shock will fit your bike. As for elastomers from Rapid Descent you can bet that Callum would'nt sell anything he wouldnt use himself, and he also does extensive testing on his products before offering them up to us. So either path you choose you'll be good to go Matt B.
I don't know. My 755 frame can only accept a new elastomer, not a shock. It has the uninterupted seat stay. I think replace elastomer is your best bet.Terry