I've been thinking about this thread, and I think my biggest problem with Mercer's ruling wasn't actually the ruling - if he wanted the monster to have the ability to negate Sentinel, he should have given IT the ability to negate Sentinel (even on-the-fly) rather than make her explain how she could possibly make Sentinel work, and then have it so that if she could not, it wouldn't work.
It's an order of operations thing that I think makes all the difference. She CAN use Sentinel. Then the Monster uses an ability of its (let's call it "Barge Through", for example, to keep moving (shoving her aside) and go where it wants to.
It winds up being the same basic effect, in that her Sentinel winds up not working as intended, but at least she doesn't feel like her play got shut down.
Note: I'm talking about a situation where the DM really doesn't want their monster to be stopped by Sentinel. I would probably just let it be stopped by Sentinel, were it me.