The pH is doing exactly what it should be doing. CO2 has that effect on it. Make sure you monitor that is you have any fish that need harder water. Mine goes from 7.4 out of the tap to 6.6 after co2.
Some people do leave the mixture out... I haven't done that, but I'm sure you will get a responce from someone who does.
The bubbles are supposed to get smaller, but only if they are moving slow enough. If the bubble rate is too fast, then there isn't enough time for them to get absorbed into the water. Since your pH is dropping, you are getting some absortion, but not as much as you could get. if you have an airline hose control (where you control the flow of air) you can attach that on the line and slowly close it until the bubbles are coming out at 1 bubble per row. By the time the get to the top, there will be smaller bubbles, however doing this may change the pH. So make sure that you monitor that as well.