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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.

You can unscrew it by hand, there are no tools necessary. To unscrew it, rotate the nosepiece counterclockwise.
The Clicklockk is held in place with the tiny screws at the side, but the noseiece is only screwed onto the body. It is the part Baader 2" Safety Kerf nosepiece with 2" filter thread # 2408156 with a regular thread (an SC-thread).

We do NOT recommend this! No filter near focus can replace a front end blocker - as 100% of solar energy hit the filter close in front of the focus position. Front end energy blocking is mandatory for H-alpha work - but not so for white light work.

Why - because in white light you could use a Herschel Wedge Prism to throw away 95.4 % of ALL entering energy - before it even reaches the first neutral density filter or dielectric continuum filter.

This is not possible on H-alpha - you need 100% of the H-alpha wavelength, but preferably zero of anything else. so - in theory - you can do this as you describe - but the filter at that heating point can break on you any second.

Also - all photographic narrowband filters ought to be fully blocked OD 4.0 from 390 nm to 1150 nm - as this is the sensitivity range of almost all CCD/CMOS-chips (most filter suppliers though do not show their blocking efficacy above 700 nm....).

So the 3.5nm H-alpha-filter is surely blocked to 1150 nm - but our front end D-ERF-filters are blocked to 1450 nm - exactly to provide perfect solar safety. Above 1400 nm most optics do not pass IR anyhow.