So my new pet sitter, who does animal training and behavioral therapy, has given me some insight into my situation. Ever since we moved the litter box the situation has continued to improve - less tension and they're spending more time in the same room. I still don't quite understand why
that litter box relocation was so important to them, but apparently it really was.
Fitz has stopped chewing the wires entirely
and I have a leather belt that I let him chew on now and then (I hold it and just let him chew on it for a few minutes). I couldn't find a leather cat toy for him, but for the small amount he chews on it, I don't think it should be an issue. The vet was unconcerned when I asked her, as long as he didn't have leather when unsupervised.
Fitz even slept in the bed with us and Daisy a few times since we moved the box!
Don't get me wrong - there's still tension in the house, but it's better. Slowly but surely.
Reg - they are littermates. They were adopted together 10 years ago. No one has ever attempted an escape, and we've never given them anything even resembling an opportunity - to say I'm paranoid about that would be an understatement. Our townhouse was only rented once before us, and they didn't have pets - the owner of the unit did have a cat here at one point, years ago, but the carpets and flooring (and paint) are completely new. They're spoiled little babies, but they've had some stressful events in the last few years - most significantly the time period where I was gone for months at a time getting medical treatment. That really changed their behavior for good - that was traumatic for them and it definitely changed the whole dynamic in the house. But for some reason this move just set Daisy off and Fitz wasn't having it, and it's changed the way they act towards each other.
But overall things have really improved - fingers crossed that it continues in that vein.
Thank you everyone for your help and suggestions.