On the breakup stuff: I suppose it depends on your Lavellan, as to how they react and deal with it. Mine just crumpled there and then on the spot.
For the most part my canon Lavellan is practical, diplomatic, treats most situations as hurdles than can be overcome through sitting down and talking about it, with a bit of underhand sneaky stuff if required, which works excellently as a Inquisitor. She's been a First for a very long time, is on the edge of taking over her clan, and so she's used to dealing with petty arguments between her hunters and local farmers. She's all business, and tends treats the world as a set of problems - nothing wrong with that, really, she loves her job and loves her clan, but it means she hasn't concerned herself with romance for a while. Solas is the first proper indulgence she's allowed herself in a long time. It's all magic, all dreams and romance in the fade, all things she doesn't usually have time for. So when she's told at this sweetest, loveliest, romantic moment, to harden her heart... eeek.
My Lavellan will work through it, as she always does, but she won't come away after the final battle without big mental scars.
On locking: I doubt it's an oversight. It's probably easier for the Keep mechanics to have one completed romance only. Even if 'complete' means heartbreak in this instance. Also, he breaks up with you very near the end of the game - nine times out of ten in real life, I doubt you'd get into a serious, invested romance in such a short amount of time. Unless the breakup was long time coming and you'd been close to someone else (neither of which are true in this case).