Well, taginfo tells me most paths do not have secondary tags, so that is not working very well.
We already have blurry areas all over OSM and there are things that get tagged two ways and it does not seem to be a problem. I do not think slight overlap is an issue. We have that with roads (=for cars) as well.
From the Wikipedia (it is worth checking, really):
Many mountain bike riders prefer singletrack over other types of trails, as singletrack is usually designed specifically for the sport
I do not know the area, I do not know what way that way is usually used but just by the look of it, I would have pathway=trail (possibly wih sac_scale and mtb_scale). It does no seem to be purpose built for MTBs. But again, I am judging only from a picture. Most trails can be mountainbiked as along as they are sac_scale=hiking, I do not see any issue here.