MP3 CDs were definitely a thing, including ones people burned themselves, but I don't think I've seen anyone use a cassette adapter. Most people put aftermarket CD stereos into their cars.
American-designed cars often had nonstandard sizes for car audio even before fully integrated entertainment systems became common. My 2001 Chevrolet has an odd, not-quite-double-DIN-sized space. If I wanted a screen for it, I would have to do some physical modifications to the dash or buy one that has a smaller main unit and a screen that sticks out from the unit (to avoid other overhanging things near it). For now, I'm happy to keep using the Bluetooth unit I put in it fifteen years ago.
What! I grew up in the UK and it was everywhere. For years you couldn't get on a bus or a train without seeing someone listening to MiniDisc. The little inline remotes were very obvious.
They stuck around for a long time, only finally being killed by the iPod.
Around 2001 pretty much all my friends had one. I owned 3 over that period (a friend destroyed one, the other me: skating falls were tough on them!)
They were easily the best format for portable music until mp3 players got their capacity problems sorted out.
They were amazing for copying music and making mixes. No one ever bought a new release on MD to my knowledge. You bought the CD and copied it onto MD. We also used them to record our band practise sessions.
Definitely not in the former USSR. We skipped MiniDisc entirely, going from cassettes to CDs to MP3 players.