> But if I open Terminal C, which is yet another size, and attach to my existing session [...]

I might be misunderstanding, but could you not use `tmux new-session -t 0` again for the third terminal?

> and when I disconnect from Terminal C, those panes do not appear to switch back to what they now should be, unconstrained by Terminal C.

Odd. Unrelated to your earlier issue, but that's not what I'd expect or am seeing when I try to reproduce is locally.