First, I applaud you for wanting to brew a 'historical' Burton and, of course, The Wind in the Willows reference. Great book!
As for your water, I wouldn't worry too much about replicating a true Burton profile. What they have listed as "Burton Water" is probably more bollocks than anything else and beers made with that much sulfate taste like crap. You would be much better off aiming for something more middle of the road than adding a ton of salts just for the sake of historical accuracy. If you know what your water profile currently is, check out "Bru'n Water" and plug everything in from there. If you don't know what your water is exactly, it is still worth using as you'll get a better idea of what 2tsp of sulfate and CaCl2 is going to do. I would be much more concerned about achieving a proper mash and sparge ph than mirroring a set water profile. Regardless, I'd stick with less than 350ppm sulfate no matter what you decide to do. Not that 2tsp would necessarily put you over that.
As for the caramel colorant, it would work out to be around 1.25 oz by weight. This can be added during the boil or fermentation or whenever. I haven't made colorant, but I do make English brewers syrup quite often and the process is very much the same and easy to do. Honestly though, I would probably just drop the caramel and instead use a few ounces of a debittered black malt or something similar for color adjustment. The beer historians will forgive you and there is so much other stuff going on no one would ever notice.
Per the mash, I don't BIAB, but unless you have a very thin mash and can't get the maize to convert, there shouldn't be much to worry about. I've brewed with 6-row malt in historical beers and again, don't feel bad for subbing it out with a cheap 2-row if you want. It's not going to make a huge difference, besides from conversion issues (if there are any).
Lastly, for the yeast, I would certainly not use Nottingham. It ferments out soo dry, you'll never get the beer to attenuate to where you want it. Wy1968 would be a fine choice, as would wy1028, 1318, and 1469. Most Burtons were rather big bodied beers, so you want something that's not going to get 80+% attenuation. If you are planning to bottle this beer, I would avoid 1968 as it can have issues with over carbonating. Actually, 1318 London III would be a great yeast for a beer like this, as it lends a slight candy-sweetness to the beer which will do very well with both the low mash temp and all the adjuncts that are going to ferment out. Hope I helped. Good luck with the brew.