Just a small detail, but: why not spend a sub menu to the undo/redo buttons, so you can save the history button completely?
I could imagine:
- press the undo button once: undo
- hold the undo button: the history pops up
- keeping the undo button pressed + move up: to scroll back in history (not showing the history page, but see what all is happening on screen). Dependent how far you move the slider, the faster you go back.
- same for the redo (of course: with advancing in undone actions).