Adelaide, I am not a medical professional so take this with a grain of salt, please, but I'd be having a very long conversation with his neurologist about his medications. I understand you have a GP in the family but they are not necessarily an expert on managing dementia. My mother's neurologist has stated time and again that it's very important to be careful with anti-psychotics, as some patients with some types of dementia and some anti-psychotics don't mix well.
When you say "drug trial," do you mean, they are trying a medication that is new (in type or dosage) to your brother, or do you mean, an experimental drug that is in a clinical trial?
I am sorry to hear about what must a very difficult situation for your family. I appreciate that you are exploring options as it must be very hard for your SIL to be on duty every day, and of course you need a backup plan for when she is ill/out of town/unavailable to care for him, for whatever reason.
I wonder if perhaps a male carer might be more acceptable than a female carer, who might be seen as a a threat (in terms of "replacing" SIL)? Oh, wait, I've just read where you say you're looking for one. Apologies.
If he won't accept personal care, and I understand that some PWD (persons with dementia) never do, then is it possible to get some help or support for your SIL, in the form of cleaners, gardening, chores, shopping, laundry, cooking, whatever? Does she have a carers' cafe or support group in her area?
I do think some people have some success identifying triggers and then finding ways to avoid them or compensate for them. It's perfectly reasonable for you to take this approach. My mother (74, Alzheimer's, no short term memory, now in a care home) gets horribly distressed and agitated talking about money and bills, for example, so I never raise the subject with her, reassure/distract her if she raises it, and have all the mail sent to me so she never sees a bill or bank statement. While this didn't solve the problem completely, it's reduced her anxiety/distress on the subject significantly. For your brother, it seems that the coffee and scone routine is important right now, and so you run with that.
I do hope that if or when the time comes for a care home (whether that's for day care, short-term respite, or a longer placement) that a solution can be find with the right combination of carers, environment, and medication, that will work for your brother. It's so challenging. (I never thought my mother would respond well to the staff in her care home, particularly in the matter of taking medications, as she was always, er, tricky about that. Definitely a "trigger" subject. While it's true that there is one nursing staff member who seems to spark delusions/conflict, generally, my mother does much better than I ever expected. I'm not saying there is a magic solution for your brother, just that I hope it could work out somehow.)
Very best wishes to you and your family.