Thank you Vlaiv, for your amazingly clear answer with the 3 Points (a post which I have now re-read several times) !
I have a question - as per your quoted text above, if we just say that we should go with 1.2"/px as the 'best' we can reasonably expect to image (with typical seeing conditions etc) then does this basically mean that (as amateurs) we can really NEVER obtain an image of that example galaxy which is greater than 750px, regardless of telescope focal length, camera etc etc?
If so, that is rather depressing...
(And relatedly: When we take many dithered images and stack them, doesn't that allow clever processing to 'generate' additional image resolution?)