1. Jim has told me once that the JRiver
YADB lookup does use Acoustic Fingerprinting, but it doesn't use AcousticID. The Album lookup on YADB is pretty good, in that it finds a lot of stuff. But the individual track lookup isn't so good. Basically, you have to take this function outside MC if, like me, you have lots of tracks that are not well identified. In my case, I inherited a music library from my brother, some of which is ripped from vinyl, and it isn't well identified. So I use MusicBrainz Picard.
2. When you use MusicBrainz, the tags it creates are written to ID3v2.3 tags in the file, if you allow MusicBrainz to do that. Those tags are visible in the Tag Dump area of the MC Tag Action Window. See attached image. With some work, including the creation of custom fields in MC, you could have MC import those tags into fields and include them in Views and so on, if required.
3. As above, I have used MusicBrainz Picard for this activity. I initially allowed MB to tag and move the files. MC would then detect the changes and update the Library. But that lead to some issues with Albums being broken up and so on. I haven't done lots for some time, but next time I do I won't allow MC to move the files, and will do that myself using the MC RM&CF function once MB has identified and tagged them correctly. I don't import MB tags into MC, so it gets a bit messy working through multiple copies of the same tracks spread across the folder structure my brother used. He wasn't very consistent. So I can't say the identification process and de-duplication process is very quick or easy. But it works... when I make time to do it.
4. Probably. I haven't read the MusicBrainz Privacy policy since I started using it. I haven't been spammed as a result of using it though. I don't tend to do any updates (edits) of MusicBrainz myself, so it is just the automatic data sharing that MB would be aware of, which is basically Albums/Tracks seen, and maybe by which accounts. They have reasonable privacy settings though.
I think the best thing to do in using MusicBrainz Picard with MC is to work out your workflow in advance, and make sure you configure MB well so that it doesn't do too much, or mess up work you have already done. Once music is correctly identified and tagged, do all your other tagging, moving, and maintenance in MC.