HI Merlin, sorry you are having problems
You said you have four cats, how many litter boxes do you have? It sounds like only one, if that is the case, you definitely need more.
As for Celia not using the litter box at all, I doubt very much that knocking her head on the door knob has anything to do with it, at all. What MAY have an affect is that she is finding pooing uncomfortable. You mentioned her scooting on the rug. Rightly or wrongly, she might have made the discomfort a direct association with the litter box and figures if she goes somewhere else it won't hurt like it does in the box. OR she might have just turned off to the busy litter box.
Whatever the case another box will help towards clearing up this issue.
You're going to have to clean the spot you referred to, with an enzyme cleaner. Even though you can't smell it, your cat can, so you need to get that smell away, or they will keep on returning and going there.
Get a black light to tell if you got it all.
You are going to a vet, that is good. You will want to tell the vet all you said here and get Celia checked for any blockage, constipation, infections, etc. Anything that might be a reason she would go outside her litter box.
I wish you luck at the vet tomorrow. Let us know how it goes, ok?