Generally speaking, Ae's audio tools are not that versatile so don't expect to have an easy time with them. about your questions - it appears that if you want to manipulate the keyframes for Audio Levels in the graph editor, you would have to adjust both handles one at a time.
two workarounds I can think of:
1. if you want to control the audio volume, it's better anyway to use Stereo mixer effects and not audio levels. this has 2 values too, only this time both are visible in the timeline so you can drag them both simultaneously (the way you could not before with audio levels) and manipulate your graph.
2. if you still want audio levels control after reading the 1st workaround: you can connect your audio levels to an expression slider: add an expression slider to your audio layer (effects->expressions control->slider control) now alt click on audio levels to activate expressions for this layer and drag the pickwhip to the expression slider. this will make the slider control the audio levels and you will have only one value in the graph editor.
Will try playing around with Stereo mixer effects.