Aquoi is right aboutuhe yeast mixture but it seems you are concerned about your tanks ph. Well lets start with what you have know. kH 7` and pH of 6.8 gives you 33ppm of co2. That is perfect. Also 6.8 is a fine ph. You should be able to grow most plants and all of the common ones. There might be some rarer species that like much higher or much lower...but you get the idea. Everything is ok so far.
In comparison, your water, wihtout co2 injection, as you would find it in local rivers, streams, etc(not accounting for extra dissolved minerals and such, which should not matter much in this case anyway) would have a pH of 7.6 to 7.8. Not uncommen and all the local plants, fish and other creatures survive in this fine. Still nothing to worry about.
You also must remember that in nature there are many and wide flucuations both wiht ph and also to a lesser degree kh and gh. Rains, flood, dryspells all cause the flucuations of various concentrations in the local water.....and the floura and fauna handle it fine
SO don't worry to much. The occasional flucuation is fine and unless you keep very delicate species you should have nothing to worry about.
It seems that your confusion about the kH/pH relationship is based on the fact that the mathimatical fomula for computiong it is some kind of sliding logrithim or something. Complex math
I would recomend checking out CHuck Gadd's site and also downloading the calculator from it. IT is acurate and extreemly handy. http://www.csd.net/~cgadd/aqua/index.htm
Read his article about the ph/kh relationship. It is pretty well written.
In short bms, odn't worry to much. Try to be consistant wiht you co2 but don't sweat it if you need to change the mix or do a wc. The fish will be just fine, as will the plants.
As for your kH rise, there must be a cause other than the BS in the yeast mix. CO2 forms and acid in water, which is why your ph drops and your co2 line getts funny at the end, and the stronger acid can cause some materials to disolve faster. If you have and rock in your tank, or shells or anything that might have a Calcium base then that is probably your culprit, The lower ph cuases it to dissolve faster and release ions that raise the kh. Remove anything that might do this and you should be fine. Also, keep an eye on your kh. if it does not change any more or goes back to normal, you could attribute it to a change in your water supply. My tap water reciently changed to a consistant kh3 to kh2. these things happen.
Whew.....Sorry for the long post. I hope it helped.