Like I said I read loads about smallbore kneeling and prone and the importance of slings. I'm familiar with jackets, keepers, handstops etc.
I agree that it's different on a course than when there's no pressure. It was enough for me to realise that, at that time, my kneeling was ok and there was a lot of work to do on sitting and standing. So I was happy to live with my kneelers ( I do ok with those ) and direct my energy towards areas that caused me more problems.
The keeper thing was another thing that put me off. If using a keeper then that gets you back to needing to set the sling and handstop up properly for a consistent position. So with all the advice about different angles on a FT course, that seemed out and just using a loop meant that the sling kept falling down my arm to the elbow as you describe.
I experimented with a few slings and even thought about a long velcro strip on my jacket from the shoulder down to the elbow. I would then have the opposite velcro on the end of my single sling. I could then just stick the velcro of the sling onto the long strip on my jacket at various positions to suit different angled targets. So I would technically have a multi position 'keeper' and the sling could be attached and detached easily in a moment. I could even have another piece of velcro on the stock ( bottom of pistol grip etc ) that could hold the single point sling up when I wasn't using it so it didn't just dangle around at the front.
So I'd put some research and thought into it, and may still return to that, but I decided it was a lot of effort for any gain, and I'd work on other areas.