Actual racers are the least of your worries on an open track, they're be behind you, around you and away without getting close or panicking you at all. You're no threat to them

Having approximate groups for level of track skill is a good idea, novices and first timers in one. Seasoned track users in another and those capable of running acceptable lap times in the top.
What usually happens is either no one wants to go in top (as it's better to look a big fish in a small pond and stay in the inter's) and blazes about in the middle group or no one wants to accept they're maybe not that experienced and avoids the novice pack. Leaving the intermediates bloated and the other 2 empty

Split sessions are great as they give you time to recouperate, 20mins driving and 40mins recovery (assuming 3 sessions) may seem a bit disproportional but if you've not done it you'll be wasted by the end of the day and you have to drive home!
On a 2:1 ratio a full day at the track should give about 6-8 sessions without any long stops and a break for lunch.
I'd personally opt for a small'ish venue, as the big tracks can become a bit dull given most of the drivers are in road level cars. the start/finish at snett is great on a 185bhp superbike but it's a touch tedious in a 45bhp mini!
