Can you move all track volume key frames down the same amount?

You've got to be careful here - this is a bit of a can of worms... If the track you are mixing is overloaded and clipping in the first place before you've done any mixing or editing at all, then nothing you can do will alter this; it's built into the recording and can't be undone (there is no cure for distortion and there won't ever be one - there are no correct reference points for repairing it). But if your recording is fundamentally sound and only appears to be clipping as a result of some processing you are doing, then you need to find out at which stage this is, and correct it there.

It's all a bit more complicated in Multitrack; you can't actually overload the system as it uses a form of processing (Floating Point) which artificially gives  a much greater dynamic range - even having a signal that goes completely into the red because you increased its level so much can be corrected. In this case though it's not distorted - it's just at a higher level than your sound device can handle. And this is where you need to be careful - is it just a multitrack overload which can be turned down so the signals don't exceed 0dB, or is it actual clipping? Until you've figured that out you won't know what to correct.

The actual answer to your question is yes, they can all be selected - right-click on one and choose 'Select All Keyframes' and then you will be able to drag all of them up and down. But as I said, you do need to check that this is really what you need to do...