If Leanther is a person's name, it must be a very rare one. The Maryland Chancery papers mention a Leanther Hughes back in 1810. This genealogical page mentions a Leanther Lord born in 1790. And that's all Google could find! (A few other instances of "leanther" seem to be misspellings of "leather".)

That Leanther is a variant of Leander is probably a pretty good guess.

I couldn't turn up any more recordings of a "Montana Song" than the 2 Amos listed.

Even "with * gentle name" turns up only 6 results with Google, none of them relevant

Who is this "John Miles" that our GUEST mentions? AMG mentions only one, and he's a British heavy metal rocker, and doesn't seem to be a likely candidate.

Anyway, how likely is it that our GUEST is still looking for this song 15 months later?