You can use fiber glass cloth and fiber glass resin
It will hold everything together nicely. It won't separate from the part later if you clean all of the surfaces enthusiastically. It's easy.
I recommend getting some disposable gloves to use while you do this.
Once you have the bits in pieces in place properly you can line the inside with some fiber ...
Some of those parts are hard to do for a beginner. Making CF look good takes a lot of practice. Still, I'll try to steer you in the right direction.
I would use honeycomb as the filler material. Cardboard gets messy quick.
How many layers should I build up and will I need a filler material?
1 X layer carbon fiber
1 X layer fiberglass
Not sure exactly what you are trying to achieve, but I'll lend my [very limited] bodyshop knowledge to your challenge.
Sanding anything to 2000 grit before basecoat is pure folly. Sanding carbon fiber is even worse: It's like trying to sand sandpaper. By nature it has texture, and most of us want the "look at this - it's carbon fiber" texture to be ...