So when clicking on a loot pile and the grids then appear below with the items, is there a keyboard hotkey to pick all of them up instead of clicking on each item individually?

There isn't such key, as far as i remember! You should pick up the loot manually.

The hotkeys are available for a number of actions like bring on screen the journal, map, spells, options, stats, inventory & you can also set some hotkeys yourself for a variety of actions like cast spell, guard, use item, special abilities and many more. This can be done from the options menu.

Hope that helps! :)