This post was very difficult to read. Although I never lived with my boyfriend, and he was never married to his other girlfriend, our position was similar to what you're describing here.
Like the other posters have mentioned, some stuff stands out:
"I don't mind if they continue to date, but with her living elsewhere, and the relationship boundaries changed to an appropriate secondary. This is the only compromise I could find" which actually sounds like no compromise at all. The idea that relationships are these things that can be managed this way, that they can be clipped and squashed into molds until they are "appropriate", is sad to me, and totally not in the whole poly spirit.
My boyfriend's now ex gf had a similar mindset: she held herself as primary, and wanted to regulate and prescribe rules and boundaries for our relationship. But it doesn't work like that. Your husband has formed a relationship with this woman, she is important to him, but you talk about her as if she is merely an element that can be adjusted as the situation requires - nowhere in your post do you mention her or how traumatic this must be for her (or, for that matter, how this must be cutting your husband up) and you even say that if she were more mature, she'd "back off"! Is this woman and what your husband feels for her so expendable?
I know I'm not being very sympathetic here, but it's coming from a place of having been that other woman. You talk about him "abiding" by your rules, you say that you expect him to be over her after a few weeks etc. but honestly, I don't think ultimatums, even implied ones like this, ever work, in any way, for those involved.
Your husband has said how he doesn't "want anything to end"... if you force him into something he is unwilling to do, it may make him resentful. Forcing all-or-nothing situations can be damaging, and you might not like the conclusions your husband eventually comes to. In my case, an "it's my rules or else" ultimatum backfired. He respected his girlfriend's "decision, and her right to make it" but once she had made it known that those were the rules she was playing by, he opted out.