Hey llNomadll nice progress!
You are welcome on the hip plate, hope it works out for you.
This warping isn't the worst, I wouldn't panic. There are ways of getting your flat surface back, depending on how thick the part is now. If it is thin enough you might just be able to simply cut a small triangle shape on each end of where you want to bend the piece back.
As for bondo, everyone has their own way of doing it. And a lot of people like to cake the stuff on the part which is probably the best way to set yourself up for days of sanding. The best tip I can give you is only use bondo in very small batches, and as the last resort if you can't get the smooth surface you are looking for.
I always resin, harden, sand, then primer before I even touch bondo. While it does make things pretty and smooth, there is quite a bit of work that is required to get it pretty and smooth. See, when you cake bondo onto your part you are basically encasing your armor part in a big, uneven, pit riddled rock. Then you have to sand and chisel at that part to get to your old shape back. And THEN, because of applying the bondo on the part unevenly you are left with tons of pits and craters that need spot putty to fix anyway or another layer of bondo entirely. I don't really understand why this is the standard, it just seems like a lot of unnecessary work.
Fortunately, we are making armor. And with this style of armor there are a lot of clean, flat faces. Though some guys will hail bondo as having that ultimate high grit finish for your paint, you can still sand cured resin down (it works better for me when I sand by hand).
Look at the part that you will be working on first and notice it's contours and flat surfaces. Notice the parts that the paper card stock is already giving you the smooth surface you need. You will notice that you can get a flat surface by merely focusing some sanding on areas where two pieces of the pepakura cut out meet. I don't know if that makes sense to anybody?
I always take time to just look at the part I'm making in 3d. Before I start unfolding I begin to pick out the areas that I already want unfolded on my page and which areas I will be ok with sanding. I already know where the paper is going to overlap and be glued together and where I'm going to need to focus some sanding. Basically it is about building smarter models.
Your chest piece had a small warp because it was a 'bubble' object without any reinforcement for the broad flat areas. It looks great, and honestly I wouldn't be too concerned with it. But if the outer walls would have bee just a tiny bit wider it would have added a decent amount of shape support.
To other 3d modelers: While building objects in bubble format is quick and easy you ever notice why building support struts with the model is becoming more and more common? It's because the card stock is not stiff enough to support it's shape on its own with these kinds of parts. You can build support struts to be included with your model, but that sort of thing gets in the way during the hardening process imo. These kinds of objects can support their own shape if you simply break it up into blocks. The ODST chest plate I built is in 3 parts, and I know that some will argue that the chest plate is one object, well that's what you can make it after you harden and sand it
The beauty is that the chest plate is very sturdy and I don't have to worry about warping the part because the individual pieces are supporting their own shape by remaining smaller and when the parts are glued together it only doubles the strength of the part while it's still paper.
I guess it more or less comes down to your own preference in building. Most people want a quick and simple paper build to start from and do the bulk of the work after. I would rather spend more time during the pepping phase to ensure that the parts look the best they can before I even start resining.
Keep up the good work man, and I look forward to seeing more progress on your project!