Does baneling building attack do splash damage to units?

Banelings usually prefer to attack units and don't attack buildings unless manually ordered to do so. When "Attack Structure" is set to auto cast, banelings will prefer to attack buildings and only go for units if there are no buildings in sight.

When a baneling explodes next to a building and a unit, the building will take 80 damage and the unit will take 20/35, no matter why the baneling exploded. If the building or the unit was targeted, or if the baneling exploded because it got killed or the player manually hit "Explode" makes no difference.

Concerning your first question, yes, the splash damage does still harm other units around it.

Banelings always do 80 damage to structures, 35 damage to light armored, and 20 to everything else in a radius.