[skies2clear]

On some of your points respectively:

1. True. Mag = focal length of primary divided by focal length of eyepiece. The larger primary will give better resolution/contrast and brighter images.

2. The shorter focal ratio (assuming a reflector), will give a larger secondary obstruction, reducing contrast performance a bit. That's why planetary scopes are usually longer F ratio (8 or higher). If you can keep the central obstruction to 20% or less is good/ 15% is better! F ratios of F5 to F7 are good allround I think for different types of objects, whereas F10 or F15 scopes are really suited more to planets, high magnification, narrower fields.

4. Exit pupil is a function of F ratio and eyepiece focal length and not objective diameter. Exit pupil = focal length (eyepiece) divided by F ratio of primary/objective mirror/lens. Also other ways to calculate it.
Yes, with a given length, increasing aperture will increase exit pupil. Calculate the exit pupil to give you the useful range you mentioned. 5mm is often considered around the max for people whos eyes are aging. As you go larger in exit pupil, aberations of the eye become more prominent too.

5. No mount can be too steady. A good Dob mount with smooth action can be used to track at near 300X manually, although being driven is better. Some say having a good tracking drive can increase the overall performance of your scope by seeing fainter magnitudes (increase of 0.5 to 1 magnitude).
If your mount is affected by wind easily, it can be an absolute pain at high mags. A great scope deserves a great mount and will make a huge difference in you actually enjoying using the scope.

6. Again, depending on the mount, it can be just fine. I regularly look at planets at 300X or higher without motor drives. As long as the mount is stable and moves smoothly, you can get away with it. I'm not saying it's as good as a motor drive though.

My 2 cents anyway...

Others will add more info I think.

Clear skies.