The hotkey for dynamite is "W"! If you hover over the menu items, you'll see their price and hotkey in the tooltip. In recent builds, we have printed the hotkeys next to the button for clarity!
Hotkeys for outposts and scrolling with arrows are interesting ideas - we'll look into it!
Glad you like the game!
EDIT: Wrote that the hotkey for dynamite is "E", but it's "W"... silly me ^^'