Okaaay. Ive found the discrepency. Its partly me and partly that old devil, Nikon vs Canon.

Firstly, I get 22.7MP, not 27.2MP. The rest of the difference is, as I described earlier, that I am cropping the image myself so starting out with FF I cut it right in half, to 18x24mm. My 45MP body is a Nikon so, even if I were to let the camera engage the APSC format, Id still retain more than 17MP cuz 17 is peculiar to Canons reduced version of the APSC format.

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I had figured from my Nikon cuz my Canon is 50MP and so acoarst my cropping that image myself to 18x24 would leave 25MP. If I were to let that camera internally engage Canons own version of APSC, that would leave 20MP. So, bottom line, I was using Nikon specs.

You're mixing up multiple technical issues.

The EF extenders (1.4x and 2.0x, all versions) do literally 'extend' into the compatible EF L-series lenses. This design makes these Canon-branded equipment physically incompatible with many Canon EF / EF-S and third-party EF-mount lenses.

But, in relation to the EF and RF mounts, the EF to RF mount adapter supports both EF and EF-S lenses, even though the EF-S lenses cannot be mounted to full-frame EF-mount bodies.

Returning to extenders / tele-converters, how third-party equipment connects to Canon lenses, it still might be a case-by-case basis.

Finally, only Canon creates 'true' EF-S lenses for their cropped sensor bodies. Third-parties creating lenses for Canon's 1.6x crop factor sensors, all these third-parties accomplish this using Canon's EF-mount that exists on the same target EOS cropped-sensor bodies.

As long as the flas unit can adequately and fully illuminate the field of view of the lens focal length in use the size or shape of the sensor does not, as far as I know, make a difference. Most better speed lights have a zoom feature to accommodate a range of focal lengths. I use rectangular speedlights, round-headed speedlights, and larger on-camera units with larger round heads and never experienced a falloff of light as long as the proper reflector or zoom setting is in place. The light pattern or beam emoted by the flas unit is reflected by the camera from the subject- the sensor only "sees" that light regardless of the primary source.

Nontheless, it cou be as you say. I learn new stuff every day. I just haven't seen anything in any of the resource literature that I checked out.