(mouse-save-then-kill-delete-region):

(mouse-save-then-kill-delete-region):
Call undo-boundary at the beginning and at the end.

(mouse-start-end): Use MODE mod 3.
(mouse-save-then-kill): Test mouse-selection-click-count mod 3.
(mouse-drag-secondary, mouse-secondary-save-then-kill): Likewise.

(mouse-kill-secondary): Do the kill in proper buffer.
Give error if in the wrong buffer.
(mouse-secondary-save-then-kill): Give error if click is not in
the buffer already used for secondary selection.
Don't switch windows; always temporarily switch buffers.