I know that in the tabletop game, you can use Slashing & Fencing Grace in combination with a small shield or buckler as they leave your offhand free. Does it work that way in Kingmaker?

I wouldn't ask, but I know that a few other edge cases don't work quite right. (Like my monk/dragon disciple can't use unarmed attacks & claws at the same time. And he can't use either to deliver touch attack spells.)

it doesn't work with shields. On the other hand, unlike PnP, it will allow you to use finesse with a weapon even if it's not finessable. Like picking finesse+slashing grace bastard sword will add both AB and damage from dex to bastards, which shouldn't be the case by RAW as "one-handed piercing melee weapon" is not a requirement for weapon finesse.