Polarizer & Neutral Density

With such a fast telescope, you should always use a coma corrector like the MPCC #2458403: Visual Version (V-1 Set), then they work fine.
The eyepieces do not correct coma and would show the coma produced by the teelscope.

The screw would be a M5x13 with a 3mm cylindric head.

Its very easy: There is the name of the item written on it. When you are behind the telescope, were the camera/eyepiece is, you must be able to read the text (text is not upside down) Means the big lens points to the filter/camera/eyepiece, the smaller one to the telescope

Maybe... The Hyperion Zoom is designed to work at high magnifications at the telescope. But the eyepiece which is included with the Regal spotting scopes usually isn't that bad, either, and the prisms inside most spotting scopes are designed to work for terrestrial magnifications, as they are seldom used at higher powers than 60-80x.
So, we can only guess if the weak point in the chain is the eyepiece or the spotting scope. The image will not get worse by using the Hyperion Zoom, but we can't guarantee if it will improve.

The 30mm extension for APS-C is calculated for a DSLR with an additional backfocus of 55mm incl. T-2-adapter. You get this distance with T-Rings which provide the standard T-2-distance of 55mm which is required by old camera lenses with T-thread.
Our MFT-T-ring provides both: The standard distance of 55mm, or a shorter adaptation when you remove the extension.
For an APS-C, you should have approximately 55+30=85mm between the T-thread at the eyepiece and the camera sensor, otherwise there will be distortions or vignetting. This is not an exact distance, but a rule of thumb. With a wide-angle eyepiece, you get get closer to the sensor than e.g. with a Plössl or Ortho.