Dennis, I'd cut that back to the ground, and sever any outlying suckers for potting-up as back-up, and wait. Which Gallica is this? I'm hoping it's not one you've collected that isn't available anymore. If so, hopefully one of the outlying suckers will be enough to get a thicket going again.
I wonder if you haven't had much RRD simply because your climate is so cold, and there aren't many "untended roses" to spread infection. It's worse in "cities", where many roses are left to linger, and no one pays attention to when they get infected. Is wild Multiflora even hardy there?
It may be herbicide, but from what I've seen of that, growth is stunted rather than "different but same size". The red foliage symptom doesn't apply to roses that don't ever express red foliage, like the pre-China European OGRs. I am concerned about the hyper-thorniness, unless R. fedtschenkoana or one of your Mosses crept into that.
P.S. Good to see you on here.