Hi, welcome to the forum!
To see embedded covers when they're available, just make embedded higher in your list of artwork sources than folder.jpg
To switch from commas to semicolons, you can do a simple search and replace in the artist field (look in the tools menu). You might find it helpful to make a playlist of all tracks with commas in the artist field, so you can weed out things like Blood, Sweat, and Tears before you do the replacement. Once you've done the change, rescan everything affected so that MusicBee will process the split artists.
I'm no help with the visualizers, sorry.