Ok, my thoughts.
1. The Nordschleife - I personally think that because of the same specs that the Minis have, any incidents (spins/crash) and differing pit stop strategies etc. would cause the field to get spread out - and not be able to catch up. So therefore because of the length of the track you may end up racing around without seeing anyone else. Not too much fun.
How about something like Brands Hatch GP (maybe 80's). It's got an interesting mix of corners, straights and elevation changes. Enough to challenge the Minis. It's also not too long that you wouldn't see another car after a spin and quite possibly cars would also get lapped. And of course, the real Mini will have raced around here (and probably still do).
2. I'd love to participate in longer races but there's usually something going on in real life that disturbs me or requires my attention. So for me at least, I'd go for your 30min duration option. I can usually manage that.
Also if someone disconnects or someone joins the room, they haven't got to wait so long until the next race. And you can then mix it up with a reverse grid for the second race.
3. I suppose it all depends on how fast you set the tire wear. For a 30 min race, if it's set to fast (or even very fast), one mandatory stop would be good and then leave it up to the drivers tire use strategy if they require more. Steady driving to conserve tires or drive fast and fit another pit stop in - could be interesting towards the end.
For a Mini, fuel consumption shouldn't be too much of an issue!
4. Yes why not, it'll add some extra interest. Brands Hatch has time/weather settings.
Although like you I'm not to sure about night racing, the lights aren't brilliant - much like the real ones.
Smiley's dawn to dusk sounds good. (You'd have to work out what time progression to use for your given race length)
To determine grid position, you may need to run a qualifying session before the first race
Anyway has I said - My thoughts.