Okay, so Counterspelling cannot be quickslotted. It must be selected through the radial menu on each creature you wish to Counterspell against.
Here's a question, though.
In PnP, you can Counterspell with Dispel Magic. However, with 4 Lesser Dispels and 3 Dispel Magics, selecting to Counterspell on a Kenku Shaman of the Tora Clan, nothing happens.
Perhaps it was just the heavy lag, but I suppose I'll find out in an experiment Tyeaan will be doing, next time I can find another arcanist to play with.
The thing was that none of Tyeaan's Dispels were discharged in Counterspell mode, even though Kenku Shaman of the Tora Clan was casting Firebrand in the combat window. (In other words, yes, it was correctly identified.)
Any input on this issue?