Blackmagic Design Cinema Camera 6K with MC-21 Canon EF to Leica L Mount Converter Kit

If you are intending to work with the Sigma 18-35mm f/1.8 DC HSM Art Lens with EF mount it would only work in the Black Magic Cinema 6k's S35 crop sensor recording mode. Black Magics S35 recording mode for the Cinema 6K is 4096 x 3072 (Super 35 4:3) up to 50 fps.

As the other comment said, there is a chance your 6k pro / 6k G2 cage will fit vertically. However, most of the cages I've seen extend out on the top to be flush with the EVF mount. There is a strong chance the top of your old cage won't line up with the Hotshoe, or at minimum, there will be an awkward bump protruding from the top of your camera rig. I'd say hang tight for new cages to come out.

The SmallRig L-Series/NP-F Battery Adapter Plate (Advanced Edition) does not provide the correct outputs to power the Blackmagic Design Cinema Camera 6K (Leica L).

Highly doubtful. Blackmagic has joined the L Mount Alliance.

Yes, the Sigma L mount lens will work natively on the L-Mount system across the board.

Yes, the new Blackmagic Design Cinema Camera 6K (Leica L), which is the camera in the Blackmagic Design Cinema Camera 6K and 28-70mm f/2.8 DG DN Lens Kit, does have focus peaking and false color tools built into the camera.

The Clip-On Sunshade from the Blackmagic Design PYXIS Monitor Kit (BH #BLPYXISMK) is not compatible with the Blackmagic Design Cinema Camera 6K (Leica L) or other Blackmagic Design cameras.

Due to the 20mm flange to sensor distance there is no room for internal (mechanical) ND. The advantage of a mirrorless style camera is that you can adapt almost any non mirrorless lens to it.