1 and 2\ I have see a scan of both uploaded. It is just a sticker. Usually the selected cover will be of the actual CD booklet shown without the sticker. But I believe both would be a “correct” front.
Basically I just treat it like any other sticker - an optional extra, with the main front cover not showing the sticker.
3\ If the digital release image includes the text as part of its cover, then it is going to be the cover. But interesting to know what happens with digital release for multiple countries as I assume there will be territories who would not bother with the “parental guidance” in their webshops leading to two alternate digital covers.
I have also seen many CD booklets to include the “parental guidance” as part of the booklet image meaning it is not optional. In these cases it is easier to upload different images for the different release countries as in CD land this is enough of a difference for a new Release.
The thing to remember is MB is about identifying a release. And that sticker is part of the release and important for identification purposes.