Gah, haven’t had to think about my tmux conf in like 5 years but here we go. This was quite the writeup, thanks for looking!
You appear to be exactly right about set -sg escape-time 0. I don’t think it’s ever caused me an issue but I am fortunate to have a very homogeneous environment and it looks like the tmux team circumvented me anyway. As of tmux 3.4:
The only time I’ve been bit by the TERM value from tmux-sensible is when logging into junos boxes. In which case I need to export TERM=xterm-256color or arrow keys don’t work (among other things). Per the docs: