Two points:
1. There is a precedence for this. After leaving office, John Quincy Adams ran for - and won - a House seat.
2. The Speaker has a lot of power. So if the Democrats held the House after Trump won a seat, they’d make it a living hell for him.
GOPe. Every house candidate would have to say they’d have no problem with him being speaker. Cheney’s toast and others will follow. McConnell will wither when Trump enters his chamber. last-SOTU address in 2023: Trump sits behind the POTUS with a big smile on his face. Priceless.
If the Democrats hold the House and Trump were elected to a House seat, he would be expelled if not denied the seat in the first place.
This is a sole power of the House, one which would certainly be exercised.