4 weeks later...
That's lyonesse holding her lovely, tolerant Icelandic gelding Stjarni. Spotter #1, cgull_ is not particularly visible. Spotter #2, my husband deguspice, took the photo.
I had a 5 minute ride, no reins, walk only. My balance is still good and I fought the urge to use my seat/legs to move the horse; that was lyonesse's job. It hurts a little, but in a good way; the gentle motion of the horse helped to loosen my shoulder. I can tell it will be a while before I'll want any connection between the horse's mouth and that shoulder. Sadly, Feronia does not neck-rein.
Mounting Icelandic-style, using a block and someone there to push up on my butt if needed (it wasn't, really) worked well. Dismounting involved right-leg-over-neck and two spotters, one on each side and the right hand one getting leaned on.
I plan to keep doing this. goddessfarmer and eventing_ponies have been working with Trump so I'll be able to be led around on him soon, too. He can be mounted from a picnic table and probably dismounted that way too.