Hello Jo, I'm Dr. Deb.
I recently came online and see that your question about Basil hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
This is obviously a very tough situation and a cat who clearly doesn't function well if his routine is changed. Therefore, I absolutely agree with you that rehoming would likely be disasterous for him and that he wouldn't thrive at all with this sort of significant change in his life.
Unfortuantely, the behavior that you're describing is not typically seen in older cats, at least not to the degree that you're seeing and not in my personal or professional experience.
I have several thoughts as to why he might be behaving this way, though:
1. Dementia which is quite similar to Alzheimer's in a human. Some of his other behavior doesn't exactly fit with this condition (such as his being tired and drinking a lot of water) but it's possible for him to have more than one issue at a time.
His need to have his life regimented with no variation may reflect dementia.
If you think this might be playing a role, I have a few treatment options which I can provide although their primiary goal is to reduce progression of the condition.
2. Hypertension can cause some cats to behave in some strange and odd ways. This condition is rarely primary in a cat but usually secondary to Hyperthyroid Disease, kidney issues or Diabetes. I assume that blood work has been done recently but if not, this this woudl be a good idea.
Blood pressure measurements can be taken in cats although stress can often muddy the results and make interpretation difficult in some patients. But, we can prescribe medication for those cats who do have Hypertension issues.
3. I always worry about a brain mass when older cats start behaving in unexpected ways. Unfortunately, it woudl take an MRI to diagnose such a problem and brain masses are not typically located in areas where's it's practical or advisable to remove them.
4. I might worry about cardiovascular issues given his tendency to become tired and dizzy. Auscultation of his chest may not reveal abnormalities but an x-ray or ultrasound might.
5. I'm not sure how to interpret his thirst in light of his other behavior. If he has a brain mass or Dementia, for example, then this behavior might reflect central nervous system changes. However, if he has problems with his kidneys or diabetes, for example, then excessive thirst and urination is often associated with these conditions.
I hope this helps although, again, my apolgoies for the delayed reply. Deb