Driftwood does not buffer, it's the opposite as tannins and other organic acids are slowly released in the water they will bind the bicarbonate making the pH to slowly creep downwards.
General rule when it comes to fish is that they tolerate higher gH if they're soft water fish but not the other way around. kH will not do anything to fish.
Forget about the pH and don't regulate your changes based on it. The americanaquarium website is a good source of valuable information. Although I will slam a bit that statement. Carbon is not a source of energy for anything. There are two sources of energy in the aquarium as far as my understanding goes: Light and Chemical reactions
Nitrifying bacteria use nitrogen oxidation to get the energy and use C for food.
ALL plants prefer CO2 before carbonates. Don't rely much on the statement they use carbonates. Some of them don't use them at all and some use it as a last resort.
In general soft water plants will not use carbonates at all. The hard water plants have adapted to use the plenty carbonates that exist in hard water.
A ph of 6.8 - 7 is good for just about anything. I'll go as far as saying that just feeding the snails a good amount of calcium can compensate for the acidity of the water.
One last thought, search on the forums for Diana's posts. I saw her replying to many threads like yours, it's a starting point to find out more about the water chemistry. You can look at my posts as well but I'm not that experienced.