> So rather than forcibly exit isearch, we could select the original window back

I see two cases that could go wrong:

1. another window is selected by display-buffer-alist.
   This can be fixed by selecting an old window back.

2. another buffer is displayed in the same window
   by display-buffer-alist.

Isearch help commands solve the second problem by simply using
isearch--display-help-action that inhibits displaying other buffers
in the same window.

Instead of let-binding display-buffer-overriding-action
in all isearch help commands, we could set it temporarily
like we already temporarily set overriding-terminal-local-map: