Well, I hope some other people can help you also but I will try to answer some of your questions.

1) Samantha had itchies from allergies a couple of months ago and I took her to the vet for it. Changing the food and making it more humid for Magoo should make him more comfortable. I also brushed Samantha more too, just make sure you are gentle around the patchy areas. It also may be stress related. I don't know about the paste or what to use for the chin acne. I hope someone else can answer that or maybe you can do a search for past threads on it.

2) I do know that for asthma, warm humid air like from a hot shower should make her more comforable. I have asthma and it has helped me and also when my kids were younger and got sick it helped them breathe better quickly.

3) I'm not sure which ear medicine you should use, is it possible to call your vet and get advice over the phone without bringing your cats in? My vet has been helpful in the past this way. When I had Samantha in for her checkup last fall she had been shaking her head and itching her ears but only needed an ear cleaning by the vet to get some gunk out. I hope I could help you a little and good luck with getting all your kitties to feel better.