The usual fact is that write-ins rarely win, in fact the only ones since Strom Thurmond's Senate win in 1954 have been term-limited incumbents that are excepted if they win a write-in vote.
Also, with write-ins, the rule sometimes is that the name can't have any spelling errors. If that is the case in Montana, anyone can sue to have misspelled ballots counted as under votes. Most people will spell his name R-O-S-C-O-E or put M-A-R-K instead of the proper spelling M-A-R-C on their ballots.
In Washington, DC this year, Mayor Anthony Williams was on course to an easy re-election when he was thrown off the ballot. He had to raise --and spend-- a small fortune to educate voters how to write in his name. Same thing with Ol' Strom back in '54.
The GOP would be best to write off Montana and extoll the fact that they didn't pull off shenanigans. It may pay off well nationwide for such a low risk in the first place.