>> Is this overriding map there just so that `isearch-mode-map' takes
>> precedence over other minor mode maps?  If so, couldn't this be achieved
>> by manipulating minor-mode-map-alist to put Isearch's map at the top?
>> Maybe that can be done buffer-locally.
>
> According to (info "(elisp) Searching Keymaps") the next keymap to try is
> `overriding-local-map`.  It can be set buffer-locally.

Actually, overriding-local-map can't be used, because it prioritizes
global-map over local maps, so e.g. in Dired typing C-o exits isearch
and calls open-line instead of dired-display-file.

According to (info "(elisp) Searching Keymaps"), the next available keymap is
emulation-mode-map-alists.  The following patch added a single line:

  (add-to-list 'emulation-mode-map-alists `((isearch-mode . ,isearch-mode-map)))

then removed all mentions of overriding-terminal-local-map,
removed all 156 lines of the monstrous macro with-isearch-suspended,
added a single line to the end of isearch-edit-string:

  (isearch-search-and-update)

and then M-e edits the search string without exiting isearch.  And it's
trivial to use isearch commands without exiting the minibuffer with e.g.

  (with-minibuffer-selected-window (isearch-repeat-forward))

And everything works fine.  It's too good to be true,
but I don't see where's the catch.