Hey there, always nice to see people experimenting with foam like myself.
I have a small piece of advice to offer with this build that will give you an end result that you will be very happy with.
I see that you have cut the tabs off of your pepakura files which is great as you obviously don't need them for foam, but what I would do is look at reference pictures of the helmet and try to tape as many pieces of the file together as you can before transferring them to foam. A quick example of this would be the center piece of the top of the helm. It doesn't actually need that joint right down the middle, so roughly tape the paper pieces together and make it as large a piece as you can. The less joints in the large pieces, the better.
I just noticed something else, from looking at the edges of the foam it would appear that you are using the knife as though it were a saw (back and forth motion). It is possible to get very tight joints (smooth edges) if you lay the foam on to a hard surface or cutting mat and pull the knife towards you as smoothly as possible. With the hot glue gun you can then press the pieces together tightly but I would then advise pressing them together from the external edge first so that excess glue is pushed to the inside of the helm, leaving you with as smooth a joint as possible.
You should be able to manipulate the foam as you are gluing it to form awkward shapes/bends
I hope this is of some use to you