A complicated project for sure, but it looks like you're doing well! Lots of patience went into this, I'm willing to bet!
For the collar, I would say to look at other collars drafted like the AoL uniform. This is an example from my Black Widow bodysuit, but it's applicable for wovens as well as stretchy knits:
You would cut two of these, on the fold. The collar would then wrap around the back of the neck, and have a slight separation at the front, as the screen-used uniforms do. You can pretty easily square the edges, like such:
If you needed to leave the space in the back to get in and out of the costume, then you would move the collar pattern away from the edge, add 5/8" seam allowance to the edge that would be on the fold (seen on the left here) and cut 4, mirroring the pattern for 2 of them. You could then put in a zipper, closing the collar easiest with a hook and eye closure. I believe, however, that the originals had full collar with a zippered front, using the flap to conceal most of it.
To make the collar sturdier and stand up better, you could add interfacing to one side, to be sandwiched in between the two layers. You can learn a little more about it with this tutorial
here. Not a lot of pictures, but gives a decent description (on short notice).
Still, a really good start -- especially all that trim, and sourcing the materials! =D