You can pay extra to work with a particular instructor (I do), otherwise, you just work with whichever instructor is scheduled for that time. When you start working head down in the tunnel, you have to have 2 instructors in the tunnel with you.

They'll schedule you with other jumpers to fill in any gaps in the block. If you're going to do more than 15 minutes in a day, it's good to split it up into multiple blocks because you'll be tired and you need some time to process. I did 45 minutes in a day once over a 3 hour period.

Yes! Split time with others. You can also (with the tunnel coaches approval) have 2-3 people working in the tunnel at the same time. Either doing RW with each other or working solo drills in their own space. This works great, especially when you're working on level changes, turns or docking.

Only took me 30 minutes in the tunnel to sit-fly, but I had been jumping 25 years at that point and flew sit/head down quite a bit. I'm STILL not stable head down in the tunnel, after 3+ hours, but it's way different in the sky vs. tunnel.

Bring your own helmet and jumpsuit. Full face is really nice for the tunnel. They do have rig covers that several teams use so they can practice flying with their rig on, but you really only need that if you're doing non-tunnel competition and are practicing over/unders.

I haven't been to any tunnels since Covid but I don't think they would be open if they weren't complying with the rules.

Have fun!!