It should work but I can't tell you with 100% certainty. I think even the CC exclusive features such as the dehaze filter should translate but just not be editable.
Correct, LR5 cannot read the new format of the LR 6/cc catalog. (Same with LR4 trying to read an LR5 catalog.)
LR6 and "LR CC 2015.x" are basically the same program and use the same catalog format.