Hi Nige,
You raised some interesting points in your post after mine. I'll answer these first in turn.
1, I am only talking about the diameter of the eyepiece barrel that fits into the focuser. It's something like looking into two straws same length, but one larger diameter than the other. The smaller straw with the smaller diameter will show a narrower field of view. Another example might help. We'll take the larger AFOV of 80deg. In the 1.25" eyepiece format, the longest focal length that will allow for an 80deg AFOV is 16mm. In the 2" format, this is around 31mm. You'll notice that the maximum focal length is getting smaller as the AFOV increases - that's because a straw will only show you so much of the sky.
2, Your reasoning of the "geometry" is correct, coupled with the refracting properties of the glass in the eyepiece. Different eyepiece designs handle things better than others. Into the eyepiece/scope equation there is also the complication of optical matching between the scope and the eyepiece. The focal plan of a scope is not a flat one, it can be concave or convex depending on the scope design. Newtonians are concave, Schmidt Cassegrains and Maksutovs are convex, refractors convex too. As a result, different eyepieces can then introduce different aberrations to the image depending on whether than can handle the shape of the focal plane the scope produces.
3 & 4, This one is going to hurt...
. After reading your posts, I'm beginning to suspect you have a dud primary mirror. I am familiar with the scope you have, and sadly very few of them have a good primary. This is more the reason why as you increase the magnification the image is degrading even more. The eyepieces you have may be 'cheap', but they will still give a reasonable image. The image will not improve with better eyepieces. The tell-tale sign is the poor image with the 25mm eyepiece, and the even worse image with the 6.5mm. I saw this too when I tested one of these same scopes a few years back. An f/4 mirror is a difficult shape to produce correctly. Sadly the company who makes these just doesn't get it, and just pumps out these junk paper weights. Sorry.
5 & 6, If you do end up changing to a better 8" scope, it should actually come with a 2" focuser, which will also have a 1.25" adapter with it. This then deals with the question of eyepiece barrel size. A slower scope will give a final image with less coma, for sure, though this is just one aberration of many, and I don't consider it a major one. If you do go for a new scope, brands like GSO, Bintel (a rebranded GSO), Skywatcher and Saxon are really good mass produced instruments. These brands have got the hang on what a good mirror needs, and have done really well to have come up with a mass production technique that makes damn good mirrors, fast and slow.
You mentioned collimation in your previous post. Collimation and the need to tweak it in a Newtonian go hand-in-hand. There are just too many moving parts that will shift ever so slightly when you transport a scope. The process sounds daunting, but it is not. It's just re-aligning the optics, a simple process that takes a little to get one's head around, but then becomes second nature. I have three Newtonians at home, and every time I set one up, I just go through the 1min go-over routine, and it's done for the night. Simple and the reward for this do-it-yourself tweak is excellent,
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Would you consider a dobsonian mount rather than an equatorial mount? Reason I say this is a dobsonian mount is much easier so set up an use than an equatorial - it is nothing more than a 'gun turret' arrangement, up and down and swing around. If you are concerned with having trouble in finding objects in the sky, don't be. With charts and the scope's finder scope this isn't a problem (the low tech end, which is all I do even after 30 years in astro). Then there are amazing smart phone apps that allow the phone to become a deep sky object finder where you sit the phone on the scope and use the phone to guide you to the object you are after.
Mental.