It's been a few years since I've been working in the industry, but let me try to address at least some of your concerns.
Concern 1: Again, I haven't been keeping up. Best way to figure it out is to look at the plug that goes to the factory HU/Box to see if it's a standard plug. Both XM and Sirius now use the same plug.
Concern 2: I do not recommend the windshield. If you read my How to Install/Replace an Aftermarket Sirius or XM Satellite Antenna
, your experience will vary. XM/Sirius signals just aren't as powerful as GPS. You drive on a steep incline or under light foliage, and there goes your reception. I can only assume rear glass installation will be worse.
Concern 3: It depends how far of a distance the wire has to travel before it can be hidden again. Usually, when we install on SUV's we ran the wire under the ceiling rather than running it all the way back down to the carpet. Also, we never installed them if people had roof racks in fear that they'd use the rack and tear up the antenna. Those had to be installed on the front of the roof which was less than ideal. Be sure to buy the ones with tiny rubber feet to save your paint.
Concern 4: Installing it directly behind the fin wouldn't look too bad. Again, no personal experience but hiding it in the shark fin would be better than under the glass windshield IMO since plastic typically has a slightly lower dielectric constant value when comparing to glass.
Concern 5: Our sales pitch response years ago was to buy a $150 piece from iSimple. How well this thing worked, I have no idea since I've never actually installed one. Just keep in mind if you buy a sat ready HU, you still have to buy the $50 control box (like the Sirius Connect) and hide that somewhere too.
I have a Kenwood DVD/NAV (Garmin) which I like a lot, but this was before smart phones. I have an 04 so steering wheel controls weren't even available at the time. My friends have had good luck with Pioneer units too.