When a druid casts Insect Plague, is the save DC of the swarm's Distraction ability 12 (what's written in the MM), or is it based on the 5th level druid spell (15+ability)?
The target isn't saving against your spell -- it saves against the creature's attack, so there is obviously no point in using 15+ability save. The spell has no saving throw itself. A summoned creature acts just like a normal creature in this context unless clearly stated otherwise.
Note that the save isn't 12, per se, it's 13+Con mod. This matters if you have Augment Summoning or the swarm's Сonstitution is otherwise changed from the default of 8.