The wall flicker thing happens to me too.When you draw a floor and erase walls just go into wall draw mode by pressing the M key.Click where your wall was and it should reappear.
P.S.:There is an excellent program called Signs II or Signs...search the forums and you'll find it.It's ree,easy to use,works great and you can make just walls with it.So I believe if you use that program to make just a wall and use wall mode to put the wall there it should leave the carpet intact.The guy that made this program did a real good job with it.I recommend it.
I am by no means an expert and I have read every forum post in FPSC and there are a lot but I learned a lot.I am by no means an expert on this and there are others who can probably answer your question better than me but I do hope this helps.
Not changing this sig until I get rich