I always think the safest place to stay is ON BOARD
It's exceptionally rare for a horse to lose it's mind to such an extend that it's going to put itself in danger and hence risk you too.
I disagree with this part, but I agree with everything else you said.
Some horses, it's better to get off and refocus their energy and mind to something more constructive than having a freak out. Other times, you DO need to ride it out. And other times, a simple one rein stop or lots of transitions, directional changes, etc.
But for your question, I always dismount my horse in the emergency dismount way. Both feet out of the stirrups, swing my legs over and launch myself away. The important thing to remember is GET AWAY. I would try to curl up but avoid bracing or being stiff or you'll get more hurt.
Don't put your hands out in front of you.. keep your limbs close.
I wouldn't try to hold onto the reins if you fall, in case you pull the horse in the direction you're going and it runs you over. But it's a good idea, if you can, to hold on if it's an open space or YOU initiate the dismount.
It's a catch 22, just be careful.
One last thing, first thing you do is get those feet OUT of those stirrups once you've decided you're going to bail. Get into the habit of doing that without looking down or kicking your horse. Looking down = BAD!!