Are they all fixed? Have they been to the vet recently? While this may be behavioral, Artemis sounds to me like she may be ill. Our Spooky gets really mean and aggressive when she's in pain. And the stress of the recent changes may have triggered a UTI in Morgyn. A UTI will almost always cause a cat to pee outside the box, and tubs and sinks are often a target.
I lifted most of this from one of my posts in another thread, but here goes.
If this were me, I'd probably get them all to a vet to makes sure. Did Cat pick something up while away? Are they allergic to flea bites? Did they get some kind of blood disease from the fleas? You're sure Cat doesn't have internal parasites now?
I'd consider separating the kitties for a little while and work on re-introductions.
If you don't have four litter boxes and you have room for that many, I'd try that. More litter boxes helps diffuse territorial issues. The rule of thumb is you should have as many litterboxes as you have cats plus one. Some of us don't have the space for it, but if you do, go for it.
You're already getting the Feliway....
So I'd consider buying Bach's Rescue Remedy (Flower Essences). Whoever cleans their water dish(es) every day should add a few drops to it/them. Dab ALL of the cats with it behind their ears, at the base of their tail, and a good large dab under the chin - this one will waft up into the nose. It doesn't always work, but when it does, it works.
Here is a link for explanations on the flower essences http://www.catfaeries.com
If you can afford it, buy either Cat Attract litter to use for a month or two - if not, buy the additive and add it to the existing litter. This is a good price for the additive: http://www.entirelypets.com/catattract.html
Do you have any vertical space for the cats? This can also go a long way to resolving territory issues. Buy cat trees or create a pyramid out of those storage boxes - cut holes in them so the cats can go inbetween and up through them - or do this with sturdy cardboard boxes (what cat doesn't love a box? We built large 3D "castle mazes" using a lot of duct tape and scissors.....) - buy boards for shelves, L brackets, and create stuff like this: http://www.katwallks.com/customerphotos.htm
And music can really help calm down stress in a multicat household - it is not just any classical music that helps. It is calm classical music that helps, specifically harp music! Here are specific music suggestions:http://www.musicmypet.com/http://www.catfaeries.com/music-for-cats.html