As one who has rolled over in an MG (with a hoop), I vote for the taller hoops. Also, I agree if you ever get rear ended, the hoop will smack you square in the middle of your head. I added some attachment points to my hoops for small luggage racks, and used the clamps to attach a small headrest as well. Also added a small amount of foam padding to the top of the hoop. Not the cleanest installation, but gives me some peace of mind. It's all a compromise.
Truck, the mirror came from a "Hot rod " supply house and clips onto the top of the Fairbourne. I think I saw a picture on the Forum where some one drilled the top of the Fairbourne center piece and installed a bolt on mirror. I actually think that might be the better solution although I've not had any trouble with my clipon. and yes I've saved the old hoops in case I didn't like the new ones. and yes it does seem to unbalance the looks slightly, but the safety considerations win out.
there is video of someone nearly rolling one of these without a helmet on. Ill have to see if I can find it. Even if its threaded in, its certainly not a crash cage and is going to have weak points that would give way in the incident of a roll-over. If I had to guess, I would assume that Morgan never tested roll over crashes in these cars and the hoops are more of an appearance of safety, rather than real safety. Total speculation, but probably a strong thing to wager on.