I did an ESB a couple of weeks ago, so my strategy is still fresh in my head. I've heard from Palmer that you don't want to emulate the water of BOT as it is too over the top. You do however, want to take a few key points away.
1. You want to have a high sulfate to chloride ratio. I don't have much experience with exactly how much, but I would say 3:1 or 5:1 would be good numbers. In any case, you want to keep your sulfate concentration below 350ppm.
2. Because you will have a high sulfate concentration, you want to keep your sodium as low as possible so that the bitterness is not harsh.
3. You want your residual alkalinity to be appropriate for the color beer you are brewing.
4. For the rest of the minerals, I would shoot for these target numbers regardless of the style (let your residual alkalinity be the driving factor):
Calcium: 50-150ppm, Magnesium: 10-30ppm
If you haven't already, I would recommend reading How to Brew chapter 15:
How to Brew - By John Palmer - Understanding the Mash pH
There's a lot of good information in there, and Palmer gives a spreadsheet to help you design your water profile. That's what I use.
To answer your questions:
1. I wouldn't do that. You don't really know what's in there, and it takes the fun away.
2. It won't hurt, but it shouldn't be necessary if you get your residual alkalinity correct.
3. You need to calculate your salts based on the amount of water you use, not the volume going into the fermenter. Your trying to emulate the conditions in the brewing city. Palmer recommends a good approach to mashing and sparging. He says to adjust your mash water with salts to get the desired profile. You then sparge with distilled water (or similar) because the salts aren't readily dissolved in the sparge water itself. You need the grains. When sparging with distilled water, the grains will bring the pH from 7 down to the desired range for sparging. You then add the salts to the boil that you would have added from sparging. Does that make sense?
4. Yes. but make sure they don't have any added ingredients.
5. Don't know the answer to this one.