I'm not scientifically minded (ex-Arts student, don't you know
) so I'm not sure that I can answer your questions tbh.
2. Bicarb in the UK is easy to find on the Baking section of any large supermarket as it's used as a raising agent. Note that Bicarbinate of soda, in the UK at least, is not the same as Baking Soda as the ingredients are different, although the Baking soda may have Bicarb as one of the ingredients. Here's
a handy calculator for figuring out how much bicarb you need to raise your KH.
3. That's something I've never heard before.
4. If the KH is stable, then I don't see why you wouldn't be. However, I believe this is one of the reason why the drop checker is becoming more popular in checking CO2. Not sure if you've seen them before, but basically a small amount of a pre-determined 4dKH solution is held in a sort of glass bubble in the tank and there's a couple of drops of PH reagent used (from a standard PH test kit which uses Bromothymol blue). When placed under the water, the checker will change colour once it reacts with the CO2 and once you get a nice healthy green colour, you know you've got 30ppm CO2 in the tank. Basically it's a permanent CO2 test by using a defined KH of 4 - because the solution doesn't mix with the water in the tank, only the CO2 you are testing with a defined reference point already. (Google imaging a drop checker will show you what I mean about the drop checker).
Sorry, not much help this!