When does Keir's romance lock in?

The scene you have with Keri after A Journey to Meridian is the romance lock-in. And it should count toward the Matchmaker achievement/trophy.

You'll be able to turn her down when you go to meet her on the Nexus after the quest. Though romancing her fully doesn't stop any other romances from happening/ locking in.

Really? I didn't especially like the romance very much, but I never saw it as creepy. I thought it felt fairly natural, especially considering how unnatural (the flirting and building up of) some of the other romances felt. Don't get me wrong, I enjoyed all of the romances after having played through the game so many times; all romanceable characters are amazing to me, but some of their romances themselves felt jarring at times.

Sorry if this seems like an attack or anything like that. What felt creepy about Keri or her fling/romance?

It was my own fault.  I was having legit a romance but also sleeping with everyone possible (I think there used to be a bug in the game that allowed it).  With Keri, it was all secretive.  The idea of hiding what they were doing - which is damn funny, considering as a reporter she wants everything out in the open - from your LI made me feel like a creep.  It was the one instance where the game is kind of like: "Yep, you're a cheater, and we're making sure you feel the weight of it."