> Currently I'm developing a patch (attached below) that allows to type
> ‘M-0 RET’ on a completion to insert it to the minibuffer, and quit the window
> without exiting the minibuffer.

Below is a better patch.  It binds C-RET to insert the completion to the minibuffer.

Regarding whether to quit the window or not, this question is currently
discussed on emacs-devel.