When a datasheet specifies output current is it per pin or aggregated for all pins?
First, these ratings are all per pin.

The question of output current sign is confusing, but it's a hangover from the days of TTL. The current being produced comes from the inputs of other logic chips, and it became standard practice to referred to sinked (sunk?) current as "positive", since that was the dominant current flow in TTL (TTL gates require much more current being sunk to produce a low than they do sourced current to produce a high, and this larger current level is what everybody had to worry about). So the sign for sourced current obviously had to be the opposite. Accidents of history, and all.