To the OP's original questions yes, yes and yes and
#4 I don't know for sure but as Al1943 mentioned you may need the 10sp chain which is exactly what I've been running for a few thousand miles now. Ultegra 6500 crank and derailleurs with 6600 shifters and cassette + 10sp chain. Works flawlessly. Can't answer about the Mtn. components though.