Penelope Williamson is very good on the romance side. Ann Parker's Silver Rush mysteries (Silver Lies, Iron Ties, Leaden Skies) are set in Leadville, Colorado in the 1870s, if you'd like a taste of mystery with your wild west.
And don't forget the wonderful The Ballad of Cat Ballou by Roy Chanslor. NOTHING like the movie, and definitely your female outlaw.

I adore Bristow, but I'm fairly sure she won't appeal to Divia.
I did track down a copy of Cat Ballou. Very different from the movie, I'll agree with you there.