I can't quite remember how I got these (I was making a cut-out there and maybe removed it badly).

Question 1: how do I remove them? ...they aren't causing any problem per se, but they get in the way of my view as I will be putting another cut-out there.
- I have tried creating a 2nd solid that covers it and boolean-union.
- I have tried to separate and delete them but that doesn't work either (or I'm doing it wrong).

Question 2: Is there a cleanup tool to remove edges that don't impact the surface geometry?

Question 3: related question: If I did create another cut-out there, is there a simple way to resize it (without removing it entirely and creating a new one)? ...I think this may be how I got these edges in the first place.

Any tips greatly appreciated!