Training to use Click macros

[Khatovar]

"/castsequence reset=combat/target/12 Insect Swarm, null"

Would that not fire Insect Swarm once, until out of combat, target changes or 12 seconds passes, and do nothing else until one of those conditions was met?

No. The point of the commas is timing. For a spell like Insect Swarm, you want it to reapply every time it is dropped {or better, every time it is about to drop}. Combat and target are acceptable reset conditions, but /click is still performing actions on that sequence, even if it is not actually doing anything, so that castsequence will not have 12 seconds of inactivity.