The first task I would do is buy a new external drive and make an archive backup. Something to tuck in a drawer in case something goes wrong with the dedup.
I would suggest don’t rely on Recording MBID alone to do the work. Sometimes you get the “same” recording slightly different. Example would be a live gig reissued. Slight different timings of the end of a track may catch you out. It can be a guide, but I would not trust it fully unless Recording MBID and Release MBID are the same.
Not really as tags may not be reliable. Read the dupguru page and it will work on the audio of a file. Using tags is only an option.
If all you have are audio files without tags, then dupguru will “listen” to them to spot the dups. And allows you to sort based on bitrate (FAQ) It has similar algorithms for images too.
I was just guessing that as you are on MB your files may well be pretty well tagged.
Both of the above apps have test modes you can run on a batch of files. See how they behave.
I’ve mainly used the image mode before to help someone out with a chaotic photo collection full of dups. Worked well once you get your head around filtering the lists it is making.
Not really relevant when you want to deduplicate. I’d trust the tool designed for the job. Picard does tagging, Dupguru looks for duplicates.
As with anything, grab a few folders of test data and try the tools out. Working with data where you know the expected outcome will mean you can see how good they are. Dupguru has a pretty geeky GUI, but it powerful once you get your head around it. Especially as you know you can mess around with it just looking at files without changing anything until you are sure.