Your grasp of both characters amazes me. I was reading this thinking about how well you understand Brian espeically here:
News alert: he not only packed up his little bag and left me, he left the fucking state.
That mix of insecurity and defensiveness he's convinced is his code.
And then Justin starts talking and I'm blown away at how well you get him.
I don't want a husband and a baby, Brian.” He shakes his head. “Not anytime soon anyway, and maybe not ever. But I do want a partner who can at least talk to me about those things. And you weren't budging an inch. You didn't give me a choice. I couldn't stay there the way things were.”
I think this was always true, but Brian couldn't hear it. And then when he realized he wanted Justin back all that fear translated in an urgency to give Justin a future now. I think your sceneio is much more doable foe both of them.

This is like what I dream for them:
I have to admit, even if only to myself, that I'm getting older, and the thrill of trawling for fresh meat three times a week is wearing off. Justin's always been the best part of my day anyway.

“We're not who we were five years ago Justin. I've already changed. So have you. I'm willing to keep an open mind.