I don't think there's a "right" footwear choice for anything with this many variables. It is grass/mud for most of the way until Milton Keynes but there's some road in that section and some mud later on (especially after Grand Junction Arms). Standard road shoes are fine, and if that's what you do most of your training in, I would suggest just sticking with them (though consider a second pair half a size up for later on). It's all about comfort and protecting your feet.
Re: directions/notes. I agree that all you need to find your way is a list of crossing points which I carry as small laminated cards - crossing points and reminder of which side of the canal you should be on on one side and checkpoints/meeting points on the other. However, the maps are necessary if you get in difficulty and need to be located as the bridge numbering sequence can be pretty illogical and you're often totally oblivious of what's just beyond the canal path.
I'll be back! DNF'd so had the fun of helping out at the finish instead of getting there under my own steam last year but completed it supported and unsupported in the past.
Tigerspaw - G2E is a really good training for this (whilst being a race in itself) as you do not often get the chance to do a long distance on the flat in Scotland. That's one of the issues with the GUCR - no variation for the leg muscles.
It's not the prettiest of routes but nowhere near as built up as you might expect and it makes for a good point to point. Start is close to Queen Street station in Glasgow and there'll be plenty around to share taxis on the morning of the race. Finish in Edinburgh is, as mentioned, by a pub and close to Haymarket. I've done it a few times as timing fits as long training run for the GUCR and running home from Weegee land is always good motivation!