One thing that may help smooth the transition is to remove your limitations on how they can behave within their relationship (i.e. they're only allowed to date twice a week).
If you express support of their relationship truly developing in its own right, just not under your roof, then you won't be asking them to give up as much. As it sounds right now, not only are you asking her to leave, but you're also asking her to limit her time with him, according to your restrictions. And you're asking the same of him.
You cleverly used the phrase "new relationship boundaries he and I put in place when I first brought this up" but in all honesty, that sounds more to me like "the boundaries that I forced him to accept." Because I'm willing to bet money that that wasn't his idea...
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."