When there is more than one recording to choose one Picard makes a metadata comparison between existing metadata and the recordings. This is very similar to how it compares the metadata when you do a recording lookup.
The criteria includes comparing track title, artist, album, length, date, but also the preferred release type and country.
Mostly yes, because an AcoustID and a MB recording are conceptionally very similar. But there are some differences. E.g. tracks with and without a long period of silence at the start or end would still be considered the same MB recording, but would get different AcoustIDs due to the length difference (if it is above 30 seconds).
There are some recent discussions about this topic: