i have got a lot of experience with chronically stressed cats. And honestly, from the report it does not sound like a multi-cat household problem especially because cats are never mentioned.
What seems odd to me is the liquid poo. A stressed cat marks its territory with piss to feel comfortable, but poo? No. Even if it was due to lacking access to litter boxes, the poo should not be liquid.
How big is your flat? How many stories? Where did you place your litter boxes? How do you feed your cats? What do you feed? Where are the drinking stations placed? Which feliway plugins do you use? Do you think that they work? It might be that you need to try one of the alternatives. Does your cat react to baldrian/catnip? How much time a day do you spent playing with each? How do the cats interact? Chasing games? Does the chased cat vary?
I'd do a blood test, check for worms, FIV/FELV, do an allergy test...
Furthermore I'd ask for
astorin Sedarom Direkt, I am not sure what the equivalent is in America. It contains Green tea extract rich in L-theanine, dicalcium phosphate, glucose, coconut fat, milk and dairy products.
You could also try "Royal Canine CALM", but I choose the pills because you can still feed quality food instead of trash 4% RC food. There are tube-foods available with that L-theanine too.
There are many options, from Relaxan (Vitamin based), to Bachblueten, Zylkenen, different essences...
You dhould choose something natural based that does not, unlike other "calming drugs", freeze the cat and rise its angst level further due to immobility.
In general I'd try to take holiday and invest the time into finding out if "you not being at home" is the reason. I cant imaging that to be honest. How long are you gone a day?