If you're looking for a really stable mount, check out this mount from BikeEmpowerment.com. It's $20, but I've been using for a year with my MJ-872 and it's a nice solid connection to the bars..no jittering or bouncing at all. Not sure how to make it better for the helmet, but the stock helmet mount seems to work just fine for me.
Yes. I use the Action-LED version here and it works much better than the O-ring my MS-816E came with. I haven't had any issues with it drooping, rotating, or vibrating once its on. I will admit that the plastic on bar isn't as stable, but, mine is shimmed slightly with rubber and makes the mount pretty sturdy. Its nice to have a quick release to leave on the bar and mount faster for quick outings.