I would say it depends on your canopy configuration. I have heard reports that a mount in the upper portion of the tailcone are prone to errors when the canopy is slid open (slider of course). Not an issue with a tip-up. I put mine lower in the tailcone to avoid that possible problem.
No, not critical. But you need to have your canopy closed during a compass swing and during runup when you cross-check the heading indicator to the wet compass (if you have a separate compass). However, if you are doing a new install, why put the ADAHRS someplace prone to errors if there are other options?
Probably the shelf behind the baggage compartment is good. Al angle from longeron to longeron. I was concerned about the cables for the seat belts crossing the shelf that might not b a factor