Yes, time got away from me. I changed jobs meanwhile, but fortunately, I am now doing more parks and trails work, so I should be able to focus on this topic again in the next month or so and come to a resolution.

Adding "access=no" is a good idea -- I'll do that in the next few days. We'll also put together a wiki post about the tag so discussion can happen there. We were intentionally trying this in a small area on a few trails to see how it would work. Thanks for your comments!

That's right. Erosion is the most common problem in hilly areas. Park officials usually want to delete such trails from OSM, but we're working toward a solution that emphasizes distinctions while not removing data. The trails exist, after all, but they're not maintained and their use can cause harm.

"Social Trail" is the general name (in the US park world at least) for trails that are not sanctioned by the agency that manages the land in question. The idea is to emphasize that those are different from regular trails or paths in a way that's somewhat similar to the difference between primary and secondary roads, so it's best to tag them in a way that will encourage them to be handled differently. This grew out of a series of conversations at a hackathon at the Department of Interior in Washington last spring (summarized at http://content.stamen.com/trailassist) and follow up conversations with park managers and the US Forest Service about how to best manage the interface of official trail data vs. volunteered trail data.