Just curious as to how this works, since it seems like a specific NPC is always who's behind them. Does killing that NPC make the kidnappings stop, or does she just respawn after a while and you're effectively always at risk no matter how many times you rescue your spouse?

No idea, mine has only happened once but i have terminated several bands of bandits on their way to Lakeview so who is to say it would not have happened again, i also regularly cull the local bandit camps from whence the kidnappers come.

i just never leave any bandits alive to begin with have yet to have any kidnappings

Kidnapping? Woah, I'm glad my assassin lives alone in Lakeview Manor. I did had to put down a few bandits there though LOL

I would like a mod that adds some guards to my estate!!!

Do only spouses get kidnapped, or can it happen to kids as well?
Also, does it happen if you're living in the city, or only in Heartfire houses?

steward and housecarl?