First more congrats on making a nicely finished car, it may need more work but it's not the finish quality...
The limited slip may be a bit more problem than I first though. Did you freshen it up when you put it in? Like new springs or whatever? I don't think you should give up on it or anything, but if it's setup for a heavier car it will be harder for a light car to make it slip. The RX7 was pretty light car ( I don't remember ), but your car is much lighter.
It's probably contributing to the understeer in your photos above too. And once you have the front wheels turned like that and you start applying the power it mostly just gets worse as you push the tires forward - then when it breaks loose it's always sudden. You will get much more used to that with just another day or two on a track. A slow track and tight turs are worst case for this too. Our autocross folks would know more about that.
All the powerful tracks cars (sedans) use various LSD and locker type diffs. These are solvable problems, but they are very real. Cars with live axles tend to use much higher front roll centers, because the live axle tends to have a high RC. That may be in your future. I'll try to find some stuff on this, it doesn't apply to me so I tend to sort of skim that stuff.
You're going to enjoy your next track days even more once this stuff starts to work better. Your going to be passing those cars that are getting by you now.
If you can get your driver's seat any lower, even if just for the track - I think you'll like that.
There's a coupe of issues with the hands. You know partly it's something can go wrong with your car and you don't get to choose when or what and that goes for other cars near you o the track. The more real thing deals with driver inputs that are unintended ( I think that's why you were holding the wheel at 12 OClock to help that ). With one hand on the wheel as you moving down the pavement and there are little dips etc. - the weight of your arm is not balanced by the other arm. So you can can get a little weave started. Then there is also a lot of air turbulence, especially getting up to 100 and above. This can also start oscillations. You have plenty on your plate, so these things don't help.
It can be boring on a straight, sort of, so it's tempting to relax a bit. I make it a mental exercise to make sure I drive as hard on the straight as I do in a turn. Make that car really be where you want it, not a foot or two left and right. Picture that line going into that next turn, think about something just stay on the ball so to speak. This gets much easier after a couple more events.
OK, so please don't let me lecture on this too much, but please don't hold on to the roll cage!
Maybe hold your shoulder belts. Try to buy yourself safety all the time with all these little things because boy when the time comes you're gonna want it all! I wear arm restraints and sort of hate them, but I do it. DIdn't use to, we just sat in our body shaped fuel tanks and loved every minute of it. Don't know if you can use arm restraints in these cars though. My formula car has a frame only an inch or two shorter than yours, but I mostly fit down inside, my shoulders hit the upper rail - and it scares me to be so high up, every single time I go out!