Polarizer & Neutral Density

Under the link Downloads you can find our PDF according digiscopie:
https://www.baader-planetarium.com/en/downloads/dl/file/id/444/product/3117/digiscoping_camera_adaptations_for_afocal_photography_and_eyepiece_projection.pdf
I´d prefer to adapt the camera this way:
https://www.baader-planetarium.com/en/baader-hyperion-t-adapter-m43i--t-2a-(m42x0.75).html
https://www.baader-planetarium.com/en/baader-t-2--15-mm-extension-tube-(t-2-part-25a).html 2 pieces
https://www.baader-planetarium.com/en/baader-tqctcr-heavy-duty-t-2-quick-changing-system-(t-2-part-06a-and-07).html
https://www.baader-planetarium.com/en/baader-wide-t-ring-canon-eos-with-d52i-to-t-2-and-s52-4289.html

And as a separate combination:
https://www.baader-planetarium.com/en/baader-2%22-safety-kerf-nosepiece-with-2%22-filter-thread.html
https://www.baader-planetarium.com/en/baader-ufc-s70--2%22-adapter-2%22-sc-thread-(f)-telescope-sided.html
https://www.baader-planetarium.com/en/baader-ufc-base-(filter-chamber)-telescope-sided-s70-dovetail-receptor.html
https://www.baader-planetarium.com/en/baader-ufc-2%22-sc-adapter-2%22-(m)-camera-sided.html
https://www.baader-planetarium.com/en/baader-2%22-clicklock-eyepiece-clamps.html

Then you´re able to fast change filters and eye pieces. Just take care that it needs about 60mm back focus and it doesn´t
allow using the zoom with barlow. On the other hand, please refer the first digiscopie PDF last page that you can reach an
enormous focal length anyway. It should´nt be neccessary using the barlow.

Ok, the optical length (or back focus) of the camera is 20mm so we have 35mm left for the adapters. From flattener to camera you´ll need following parts:
https://www.baader-planetarium.com/en/m48-fine-adjustment-rings-(03--05--1-mm)---aluminium.html 1mm
https://www.baader-planetarium.com/en/m48-fine-adjustment-rings-(03--05--1-mm)---aluminium.html 0,3mm
https://www.baader-planetarium.com/en/baader-ufc-s70--t-2-adapter-t-2-(f)-telescope-sided.html 1mm
https://www.baader-planetarium.com/en/baader-ufc-s705-extension-5mm-with-s70-dovetail--dovetail-clamp-screws.html 2pc=10mm
https://www.baader-planetarium.com/en/baader-ufc-base-(filter-chamber)-telescope-sided-s70-dovetail-receptor.html 13mm
https://www.baader-planetarium.com/en/baader-ufc-s52-dovetail-camera-adapter-(eos--sony).html 2mm
https://www.baader-planetarium.com/en/baader-wide-t-ring-canon-eos-with-d52i-to-t-2-and-s52-4289.html 8,3mm

This is in summary 35,6mm because filters lay out the focus to extra focal caused by refraction through the filter. Rule of thumb is a third
of thickness of filter according to 2mm thick filter is roughly 0,6mm.

Yes! Back focus length of the Canon EOS-R is 20mm and the EF-EOS-R adapter is 24mm. Plus this 11mm long Canon EOS EF bayonet ring gives you the needed 55mm back focus.

Yes! The optical length of the camera is 44mm and this bayonet ring is 11mm so you get the needed 55mm back focus.

Do you have any extensions in use which are required to reach focus with an eyepiece? The MPCC is designed to work with a DSLR, that's why it has a working distance of 55mm. To reach focus with a DSLR (or a camera with MPCC), the focusser must be moved farther inwards than with an eyepiece. On some Newtonians, this is made possible by some extension tubes, which must be removed for photography - otherwise, the focuser tube would be in front of the main mirror.
On the other hand, there are Newtonians which are designed strictly for visual or photographic use, both by focus travel and size of the secondary mirror. With these telescopes, it may be that they can't provide the backfocus needed for a DSLR or a coma corrector, although you can reach focus with a camera which requires as little backfocus as your ZWO.
Please check the telescope's manual if there is any information about attaching a cmaera, especially a DSLR.