Thread: Inappropriate marking:One step forward, two steps back

awwww poor 3 of them!
To me, he cries when he pees, he pukes yellow stuff, it does say something physical, not coocoo-ness, no? did he have blood tests done? (I think you said above but don't remember) - may be just in case try to get him on that special diet...
but see, he does attack Samson but not Rose. Something in Samson's smell may be sometimes not all the time makes him anxious... as you say, sometimes Samson smells differently. May be in that case Merlin doesn't really recognise Samson as his usual we-get-along-fine companion and lashes out at that unknown cat on his territory? kind of "where's my Samson and what have you done with him?" kind of thing... Cats do rely on smell a lot....
What I would do is try to take note of what he eats, you know a journal, and see if you find a correlation with the "episodes" - if he pukes or cries more when he's had such or such food then you have your culprit, sometimes it takes a day or two to get through their system so you might not remember "oh yesterday I gave him fish and today he's crying, and the last time he cried he had fish the day before" wouldn't it be great if it were that easy? lol And also try to see how Samson smells when he attacks him (though a smell to you is not necessarily the same smell for Merlin, it's kind of pheromony and therefore not accessible to our noses, but it would probably be a start?) and there are dry shampoos also may be try that instead of wrestling that alligator (I am sure Samson agrees, it would not be clever and why would you wanna do that! lol) if you do, don't forget the face, cheeks, and such, that's where they get the pheromony/identification smells... can't hurt to try... and once again wouldn't it be fabulous if it were so easy! And finally, I would definitely order some of the Zylkene that Jacky talks about... personally I would not like the idea of giving valium to Neevie without knowing for sure if that's what she needs.
Sorry I hope this doesn't sound like "I'm an expert do as I say" - I just share your concern, I have had a cat with a difficult health in the past and I do wish someone had shouted ideas at me to try and understand what was wrong with her, rather than have the vet cut into her straight away with disastrous consequences... :-(