Using Ctrl-P to get the most previous string completion match, selects the next, not the previous match.
I'm not seeing this locally. When I use CTRL-P it will navigate up the list and CTRL-N will go down the list.
The one known bug is that when the selection is on the first item CTRL-P won't wrap around the list to the bottom. Is that the behavior you're seeing? If so there is already a bug tracking this (one you filed IIRC).
Yes and no - the first matching previous text occurrence won't be selected/suggested and the first (not matching) suggestion will be selected, so that Ctrl-P won't work, because it does not wrap to the bottom of the list.
But more important is to have the first matching text occurrence should be suggested at the top of the list, not matter if searching for the previous or next occurrence.