Collision of volta and chord symbols - how to resolve?

Right click on Volta> Select all similar elements;
Inspector: Adjust the vertical offset.

I can't seem to be able to move individual chord symbols…

You should be able to. If you're having trouble clicking on it, hold down the [Shift] key and drag a rectangle around it to select it, and then use the arrow keys (or the Inspector) to move it.

I would use a combination of settings in this case. First, make the Chord Symbol Style lower down (in Style > Text… > Chord Symbol). Then, make the Volta style higher up (in Style > General… > Hairpins, Volta, Ottava). Finally, I would move just these specific chords a little bit to the right.

You can definitely move individual chord symbols. Just click and use the arrow keys, or the offset fields in the Inspector, or even simply drag if you don't care about precision. Sinilarly, you can resize voltas - double click, drag the handles.

Rather than do this for each collision, though, you should consider changing the default positions of these symbols, as suggested above. Lowering the chords and raising the volta will indeed help. It really wouldn't be necessary to extend the hooks, but you can set those defaults as well in that same dialog if you prefer that look. Then all you'd need to do is push the chord a little to the right, or the beginning of the volta a little to the left.