That's the way I would do it. I would consider it foolish to put in a temporary COA for such a short time under the circumstances of the OP.
My only concern with this is, what if the new house purchase falls through and the time at the in laws ends up being more like 4-5 months? I'm probably overthinking it, but it seems easy/cheap enough to send the address change form multiple times and not be out of the 30 day window.