1) Did Mitchell have the power of subpoenas?
No.
2) Were people required to sign affidavits before giving evidence or reqiured to take an oath under the threat of perjury?
Sort of. Brian McNamee, who is Clemens' accuser, agreed to cooperate with Mitchell as part of his plea bargain with the Feds. McNamee was given immunity with respect to his statements to Mitchell provided he was completely open and truthful. A Federal prosecutor was present at each interview to assure compliance with the plea agreement.
3) Where is evidence to back up these claims?
Although I can't recall whether McNamee provided documentary evidence specific to Clemens, Mitchell's report has an extensive appendix that contains documentary evidence to back up many of the allegations, including canceled checks, notes, FedEx receipts, etc. I also note all of the named juicers were given an opportunity to respond to the allegations and all but a handful refused.
4) Andro was neither illegal nor a banned substance when McGuire took it. I wonder if the concept of ex post facto means anything to any one?
Mitchell acknowledges that in the report. But that vast, overwhelming majority of the accusations refer to stuff that was illegal and/or forbidden by MBL rules at the time of use. He uses McGuire and Bonds as evidence that MLB, from the Commissioner's office right down the chain of command, buried their heads as the use of juice became widespread.
You saw the report.
Mitchell timelines it back to 1991 when Fay Vincent banned steroids in a memo.
Technically, MLB should have stated an opinion when McGwire was using andro openly.