Good observations/advice here !
Just a thought on the "should the QR point to .mobi or the full site" item (in the case were "website doesn’t do any user-agent sniffing" to detect a mobile client)
When do you NOT scan a QR with a mobile? OK, I know I can install a QR reader that would use my computer's webcam, but in 9 of 10 (10 of 10?) cases, wouldn't people scan with their mobiles?
So, give them the .mobi !
(and possibly a "switch to full site" link somewhere on that site, for that 1 out of 10...)