next question: how would one go about betting on Korean residential mortgage credit spreads as an American retail investor?

Thats not the good bet - based solely on this information - the landlords would be fine on their own property, they would have lost a portion or all of a prior renters deposit in some other speculate bet instead of just letting it sit in a bank account, new renters deposits are paying for the ability of the landlord to pay the prior renters

Of course that prior speculative bet could be in mortgage(s). You would need to look at other weakness in that economy to rationalize whatever bet you want.