SNPtheCat said:
Let's say you leave the handling to a third party. How would one determine who is cheating, and what standard would one party have to reach in order to show that the other party has cheated? If you set the standard too high or too low, it either becomes ineffective or open to abuse, or both.

Well the most professional approach would likely be a hybrid or a tiered system. For example, the agreement could specify that clear, provable physical infidelity (the "high" standard) triggers the full payout. Other, less provable forms of betrayal could be handled through a different mechanism, perhaps a mediated resolution or a partial dispersal of the funds.

Basically, the act of having this conversation and reaching an agreement on what constitutes a breach of trust is one of the plan's most valuable, albeit challenging, features.