Welcome to the forums Mark and yes, it certainly is a rabbit hole that you never come back out of
I'll start with the most pressing first, your 10" newt with patience and understanding will work on an EQ6 but it really is working at its limit there. Disregarding the pains that come with working at that extra focal length, pushing so close to the EQ6 payload limit can make it a bit of a nightmare to get working right.
So, not that that is out of the way, I'd say you're on the right path
Whether you'd want to start with the EQ6 or the EQ5 is up to you depending on where you see yourself moving. The EQ6 does have a larger payload limit so does future proof yourself a lot more, the EQ5 is a brilliant beginners mount and both will be just as capable starting with an EQ80.
The Black Diamond and standard ED80 are identical telescopes yes. They may come with different accessories and even potentially be a different colour but they are identical telescopes. As to which one to pick, if you're only looking at doing astrophotography with it, which ever one has the better focuser!
If you are happy to limit yourself to 60-90s exposures then you don't need to auto guide, as long as you get a good polar alignment that is. For most DSLR photography in a light polluted place, you're not likely to be able to go past that anyway. Guiding is of course preferred for a number of reasons but you don't have to jump into that straight away if you don't want to. Just remember that it IS where you are going to end up
In time, you may find yourself wanting to upgrade from an ED80. Being a doublet refractor it will exhibit some star bloat as the blue/violet part of the spectrum isn't being corrected for but there are a lot of 80-120mm triplets that will sit on an EQ5 or up to 140mm on a EQ6 (some use a Sky Watcher Esprit 150 on them, very close to the weight limit there).
An 8" newt will work fine on an EQ5 but there is a significant increase from 8" to 10", then again from 10" to 12"! The lighter carbon fibre 10" F/4 newts are fine on an EQ6 but they're a lot more expensive than more standard 10" models.