You're making it too hard. Read here:
Adobe Premiere Pro * Recording audio mixes
And here:
Adobe Premiere Pro * Adjusting volume levels
If you decide to use volume keyframes instead of writing track keyframes with the audio mixer, there's no need to chop up the clip(s). Just set a keyframe at the point where you want the volume to be constant, and another keyframe with the same value at the point where you want to change the volume. Then set a volume keyframe a few frames later at the level you want for the next section. Lather, rinse, repeat.
Jeff