I think I'd simply use RET as the key. Since the preview only appears
after the user had pressed C-M-i already, this should conflict with
the regular usage of RET.
I'm not sure I completely follow, but I do agree that RET is another
good choice for "accepting the suggestion". OTOH note that the
completion preview appears automatically, without you pressing C-M-i, so
I feel that it would probably be better to leave RET alone for inserting
a newline. Another good candidate, IMO, is TAB. But I do like how with
João's suggestion we don't rebind any key, which makes things somewhat
simpler. If a user does want to use RET or TAB in order to insert the
suggestion when the preview is visible, they can bind
`completion-at-point` to RET in `completion-preview-active-mode-map`.