Hello there!
I'm currently handling the BHG/MW costumes for local approval, so here's my take on your questions.
1. I'd probably stick with the cotton blend as the CRL specifically mentions it. Dickies coveralls slightly modified would be a fairly cheap and easy way to get that. Leather probably wouldn't achieve the desired look, as least according to those who designed the CRL.
2. At first blush the jetpack looks similar to Jango's Arena Jetpack, and the Dented Helmet does have some templates for that here:
https://www.thedentedhelmet.com/forums/threads/boba-and-jango-packs.10051/There are a few services that do print out jetpacks for you, I've seen anywhere from 90-130 based on size from some Etsy vendors like this one:
https://www.etsy.com/listing/1025942821/jango-arena-jet-pack-kit-full-size?variation0=2051638645I haven't done business with them, so I can't speak for their print quality, turn-around times, etc. Just providing a potential source! 3D printing also typically requires quite a bit of sanding, filling, etc as well.
3. I don't see anything specifically requiring the jetpack to be able to detach from the armor. I know for Boba there are straps that come out of the armor and fit onto a harness under the vest. That's a bit more comfortable than just having all the weight hang on your back with the armor. That being said, Boba's latest Jetpack does attach directly to the armor, so long story short, whatever works best for you as long as it is flush against the armor.