I've done loads of days in all sorts of cars.
1, I've never felt that there were any plebs in attendance at any event I've been to, organisers are quiet strict on inappropriate behaviour on track. Its not the scene usually for chaved up corsas, Open pit lane days tend to give a bit more flexibility and once the rush is done at the beginning you go out when you please. Sessioned days are ok, but if you end up with a problem in your session you then miss out. Having said that it does give you and the car time time to cool off.
2, Up to you, I've not bothered, but its your call.
3,Yes, covered arms generally to.
4,Depends how you drive but it will wear them heavil, I would make sure you've got at least half on your pads, and the tyres are in good nick.
5, Obvious stuff, make sure fluids are topped, pressures are correct, tyres and brakes as above, and you empty the inside and boot. Stuff flying round the cabin isn't good!
6, Group or session days, tends to group by the description you note however if you're an novice but have a quick car, like the F, I'd put yourself in a class above other wise once you get into it slow stuff will constantly be in your way. However in most cases the organisers will switch you if you are in the wrong group.
Hope that helps. Where are you thinking of going?