The lack of underside body panels and the compressed 16:9 format into 4:3 of the video make Baggy Joe look a little like a racing 2CV
A big congratulations on coming 3rd! That's very well done, Andy.
About roll: The only way to reduce roll is to lower the car (reducing the forces acting on the suspension), and/or to stiffen the suspension. Unless you go to extremes and change the suspension layout completely, but I guess that's out of the question...
But the fact that you go so well through the corners (despite the roll!) indicate to me that your car is performing quite well. How did it perform compared to the Miata's for example? It's quite obvious that you have a very good feeling of what you are doing, whereas the Miata drivers are either inexperienced or have less driver feedback from their cars - or both?
About understeer: Yes grip generally increases with increased tyre pressure (until a certain point) but the best way to set tyre pressure is to take a temperature measurement across the tyre surface. It should be even. This can be done with an IR thermometer. High temperature in the middle indicates too high tyre pressure (tyre bulging out), higher temperature on the edges indicate the reverse.
Increased camber can help, but the IR thermometer will help you there too: High temperature on the outside edge of the tyre and you should increase (negative) camber. And vice versa.
But there's one more way to reduce understeer, and that is to
transfer load to the front of the car. Ballast is one way to do it, but it adds weight. Reducing rear weight is another, but could become expensive.
No, the way race car engineers have for years balanced their cars is by setting up the anti roll bars: Making the rear end stiffer, transfers load to the front wheels and reduces understeer. Make the front end stiffer, and you reduce possible oversteer. Etc.
This works by transferring load from the least loaded wheel, through the chassis, to the diagonally positioned wheel in the other side. So if your left hand rear wheel is very lightly loaded in a tight left hand corner, a stiffer anti roll bar will aid transferring load to the right hand front wheel through the spring/shock absorber and the cahssis. THIS will reduce understeer.
I think you may find inspiration on how to make an adjustable roll bar on this photo of a Matra F1 car. The roll bar is the silver bar starting next to the rear wheel in the front. The bracket on the bar is attached to a rod connected to the lower A-arm on the suspension. The bracket can slide front to rear of the roll bar to tune it:
New rear shocks should also be considered, as in the dynamic environment, they will also be transferring a lot of load from your cars' rear to the front.
The above is just theory, I'm not a race car engineer (I would have loved to be though!), but I've studied the subject for fun, and I've promised my second-oldest son that we will buy ourselves a Formula Ford to do some classic racing in - and to be successful in such a beast will require a lot of suspension tuning (while engines aren't allowed to be changed).
It's great to see you emerge from "your depth"!
Cheers,
Anders