Hi Tim - welcome to Purrs to you and the very splendid Mr Douglas Cat, Senior Kittizen and long time companion.
Thanks for giving us a comprehensive overview of Douglas's behavior and characteristics, as it really helps when issues like this crop up.
There's been a lot going on in Douglas's life of late, hasn't there? This can be quite tricky at any time in a cat's life, let alone when they're most definitely "pensioner cats" He's lost his previous companions, and now has found them replaced with one at least who has exerted his own character to the point where Douglas has suffered physical harm, and needed veterinary treatment. he must be feeling pretty fed up. Yu can imagine how you'd feel if your family suddenly moved three other people into your home, telling you that things would be fine, and then one of them rounded on you and gave you what for, sending you to the Doc for treatment as a result. And that's how Douglas's world has been affected.
Introductions between new cats can be tricky to manage. Were the new cats confined to one room initially and gradually introduced with site and smell swapping, or were they just brought together and left to work things out?
It sounds, having had Douglas to the vet, that he has the sort of health issues you might expect with older cats, such as his heart murmur (although I appreciate he's had that many many years) his thirst and his vomiting (often an indicator of thyroid problems). The fact he doesn't seem comfortable putting his bottom on the ground could indicate pelvic problems, arthritic changes, or an abdominal problem, and I think it's worth raising this again with the vet, especially after the blood in his stools (though that may just be a stress reaction too).
It does sound as though his current actions are behavioural, and that he's stressed out by the changes and the new arrivals. It looks like his peeing and defecating is his way of marking what is "his" - ie your things and those of what he considers as "his people."
Are you continuing to use Zylkene, and if so, how often does Douglas receive it?
We've just recently experienced inappropriate peeing with one of our two semi ferals we took on in November. They share our home with Moray, a young, laid back NFC look-a-like cat, who is very chilled and good natured. They all rub along (wouldn't say Moray's thrilled with his new companions, but he tolerates them, and I'm sure when they're less hooligan like, they may actually get along very well).
One of the Gurlcats began peeing on the snuggle chair in our bedroom, as she was marking it as hers and my OH's. I was using it at weekends, and therein I think lay the problem. The Gurls are very territory possessive, due to their rather wild beginnings.
I can tell you what we've tried, and what has worked for us, and you can maybe see if it works for Douglas.
First of all, if you can, give Douglas somewhere that is completely his, where the other cats are out of bounds. Your bedroom would be ideal to begin with. This will be exclusively his place. Confining the cat to one room may seem cruel, and I understand why we think that way, but often with a stressed cat, it reduces his space to a space he can defend comfortably and without having to worry. This is where the unpalatable bit may come in. You're going to need a few litter trays. And put them around your bedroom - maybe three or four. One of them should go on your bed, so you may want to revert to the plastic while you try this. Put his food and water in the room for him too. This will become his domain. I would clean the litter boxes as often as you can (this will also help you detect whether there is anything further amiss with his toileting, such as a return of blood in his poo.).
If he uses the litter trays instead of the bedding etc, that's a good first move. Leave the trays in situ for a while (obviously removing them from the bed at night when you go to sleep, but leaving the other trays - I hope your bedroom's a reasonable size to accommodate this!)
We used (and still use) Pet Remedy plug-ins. On the whole whole we've found Pet remedy to be more effective than feliway, but I think that depends on your cat as much as anything else. We put plug ins upstairs and downstairs. We also bought Pet Remedy spray (around £20 from PAH but you can buy cheaper online) and sprayed bedding and furnishings and our hands when handling the girls.
We also cleaned the soiled area with a special cleaner - a wonderful lady on this forum called Rosella mentioned to us a product called RX66 - you can only buy it online so far as I know, and we paid about £12 plus P & P, but it's an enzymatic cleaner. It was fantastic for cleaning the soiled chair, and it completely removes the smell of pee and poop if you follow the instructions. Obviously you can wash bedding on a hot wash or add an antibacterial to the wash, but on carpets, your son's sports bag etc, try the RX66. It's very good.
After we'd cleaned the soiled areas, we began to put small plates of food down on the affected chair, working on the principle that cats generally will not urinate or defecate in the vicinity of their food. You may just want to put a spoonful of food or his favourite treat in dishes at the various favoured pee and poo spots, including the bed. Hopefully, he will then begin to disregard the bed at lest, for his toileting. (Baby steps are everything when you're trying to work with a traumatised cat).
I would still let him go out when he wants to, but make sure he has access to his room as first resort of safety, and that the other cats do not enter that room when he's not in it.
This has helped change the behaviour of our Gurlcat, and she's not now using the chair as a litterbox, and (touch wood) she hasn't chosen a replacement. We had to persevere for about 5 weeks in order to see a marked difference.
Does he still climb, or does he like places to retreat into? If so, you could try getting him a cat tree or a "den bed" that he can use when he's elsewhere in the house, assuming the suggestions I've made above show any improvement in his behavior.
If you can try this for a week or so, and monitor any alterations in his behavior, then you can consider planned reintroductions to the other housemates.
Do the other cats get playtime? Does Douglas get playtime? Even older cats enjoy some "me" time with their favourite people. Maybe you or your son or other family members could play with him with a rolled up paper ball, just flicking it back and forward gently, to see if he'll engage. or try him with a wand toy, and mimic prey behavior, to see if he responds. Fifteen minutes twice a day should be good. If he'll respond to your play overtures, it will give him an energy release, help to stress bust, and of course, let him know that you still care for him as Numero Uno puss-cat.
I hope some of these suggestions will help Douglas (and your family) get his equilibrium back.
Liz, another forum member, has a large number of cats who've all learned to rub along together, but she has some good tips and tricks for easing the way too.
And I see the lovely Rosella I referred to earlier has beaten me to it in terms of posts.