The built-in library saveplace.el contains a commentary that states:
;; Automatically save place in files, so that visiting them later
;; (even during a different Emacs session) automatically moves point
;; to the saved position, when the file is first found. Uses the
;; value of buffer-local variable save-place to determine whether to
;; save position or ...
By default, Emacs keeps just one older "backup" of every file you edit. If you edit a file called foo, then the backup file is called foo~. Whenever you save a new version of the file foo, the previous version is saved as foo~, and the previous backup file is discarded.
You may request that Emacs save multiple backup files, thus yielding a version history ...
C-M-i in Isearch is pretty lame, as you indicate. In general, in vanilla Emacs completion is not incremental, in the sense that it does not continue completing (updating the possible completions) as you type (change your minibuffer input).
If you use Icicles then you don't have this problem - completion is incremental, by default.
In particular, for ...