I’d suggest you also do some case law research on how the MT courts have interpreted the “adverse” or “hostile” requirements for prescriptive easement. I’m dealing with an easement dispute on one of my properties in UT right now. In UT, the case law supports that “adverse” can be assumed, and as such the burden of proof shifts to the defendant to prove otherwise. That proof would need to be some type of formalized agreement showing permission was formally granted. Which is why I suspect the property owner in your case is now asking people to call for permission because it will establish that.
Irregardless, I think USFS is the only party with standing to bring a lawsuit here. Just because you’ve used someone’s property to get from one piece of property you don’t own to another piece of property you don’t own does not give you standing.
I like the idea of engaging your local LE. They may tell you “it’s a private matter between private individuals” and not want to get involved (which is what I was told), but it would still help to make them aware. They might have insights into other people making similar claims. Not to mention that it would give you a chance to. Are your case to them before the landowner does. They would probably appreciate you being proactive about it.
I didn’t see it mentioned here, but I assume you’ve checked the county plat maps to see if the road is on there? In UT, that’s really the litmus test for whether a road is “public”. The fallback is then recorded easements, but easements have issues with clearly defining who the “grantee” is. Unless it specifically says that it’s “the public”, then it’s likely restricted to a named grantee. If there’s no formal plat recording or easement recorded, then the only way to make this officially a “public access” is for USFS to file a lawsuit or negotiate such with the landowner and then record the easement as a result of that process. LE is generally not going to attempt to enforce a prescriptive easement. They’ll only enforce it after you’ve sued over it and won - which would result in an official court judgment and, hopefully an official easement being recorded.