The little bit that I've learned in researching roll cage design is that you have to imagine the forces that'll be acting on the cage.
In my mind, this is the strongest option (ugly, but strongest). In the event of a roll, the downward force on that tubes will be distributed across 2 different tubes. Red arrows represent force.
This design may look cooler, however, in the event of a roll it is not as safe. The downward force would be distributed across one tube, horizontally. It's just going to bend your door tube. Would you survive a roll with this design? Most likely, but your cage would get bent/tweaked after a hard landing.
This is what I was talking about in the rear. I don't think the current design is very strong in the rear. Let's pretend you roll. The amount of force going straight down may be too strong for the cage, and bend that corner in (the same reason you're worried about the front corner of the cage).
In order to strengthen the rear roof, I recommend adding the green bar. It will help distribute the downward force in a roll.
This part also worries me.
Think of it as a giant cheater bar on your weld. Just like how we throw a pipe on a wrench when we need more leverage to break a stuck bolt, this design puts more leverage on your weld. The longer that pipe, the more leverage.
If it were my vehicle, then I would cut off the rear part behind the B pillar, and just add a triangle like this (green bar). You'd have to solve the issue of your shock hanger, but that'd be easy).
This picture below looks like overkill to me. I can see how the horizontal bar would help on a side impact, but I don't think you'll see enough force on those corners to bend them in towards the center of the vehicle. Technically, it is stronger with those two ^ bars connecting to the center of the roof. I just don't think the rear of your cage will ever see enough force to require the ^ shaped bars. I guess I'm trying to keep in mind that you want a lightweight rig, and that this would be adding weight to the tallest part of your vehicle. (granted it's still not tall, but you get the point.)
I hope that didn't sound condescending or like I'm a know-it-all. We all know I'm not. I'm sure you knew most of this stuff, I just wanted to explain my reasoning.
I wish I could find that amazing article on designing a roll cage. I have looked and looked but I can't find it anywhere. It explained all this stuff MUCH better than I did.
