it seems to me like a quickly moving target.
Know what you mean.
It's quite simple actually. Your computer is just a machine, and Mint is just a software that helps you work on it. Just as you would need a web browser if you want to browse the web, you need other things for other functions. For example, you need a file browser to browse your files, and you need a photo manager if you want to browse through your photos collection.
And you don't have just one software to do this. For example, there are a few different web browsers that you can choose from, and the same is the case for your file browser and photo manager too. You may not know this but if you don't like the default file browser you can install another one.
This concept extends to your desktop environment too. You can choose from gnome, KDE, xfce, lxde and a few others. The desktop environment is what gives you the working environment that you are accustomed to. Gnome and KDE is one of the higher end ones in that you can do a lot with it, the desktop cube being one of the best known examples. xfce and lxde are much lighter and offer a faster environment, but no extra features.
It may all sound a little confusing initially, but the more you read up on it, the more everything falls into place. It looks like you've been hunting around and gotten confused with different things. My suggestion would be to take your time. Mint is remarkably robust, and even if you do manage to screw it up, you can always reinstall. Just make sure that you have everything backed up.
As the old saying goes "Rome wasn't built in a day" so take your time. It'll all come to you.
What is simplest (or 'official') pathway to follow?
My suggestion would be to keep following the forums. I learned most of what I know from there. I see a question, and then read the answer and if I don't understand it I search around. As time goes, I learn more and more.
What should be on my system: Gnome 3|MGSE|Update 4| ? (Remove all else? MATE? Cinammon? Gnome 2)
It all depends on what you want. Gnome 3 is the next version, up from gnome 2. MATE is another name for Gnome 2. Cinnamon is a Mintified Gnome 3. They are all desktop environments as explained above. Update pack 4 is for another version, of mint, not the standard one.
When I logon : Do I want to select Gnome, Gnome 'classic', Gnome no effects? Something else?
Once again, it depends on what you want. They are all different desktop environments and you should try them before you make up your mind. If you are unhappy with them or want to try something else too, you can install xfce, lxde or KDE on to your regular install. You just choose which one you want before you log in. There is no problems as such with doing this except that you'll have different applications for each DE that will clutter your menu. For example, KDE and Gnome are not usually compatible, so you need a different app for each function. mate, gnome 2, 3 and xfce use most of the same libraries and the applications can be interused except for a few.
If possible to just list in one place (not even explain) what should be running, I'd be happy for now
If everything sounds confusing, go with MATE. It is not new, hence more stability. Cinnamon too is very good and the Mint developers have worked hard at ironing out all the bugs, but you may come across a couple here and there. Other than that, most of the applications you will need for a normal home user will come default in the DVD version.