If there's no Plan B, then the first rule is "Make Plan A work!"
Wait! Isn't there always a Plan B somewhere?
A general officer once told me: If there's no Plan B, invent it!
Insure for failure?
Maybe you can insure against the failure of Plan A. Perhaps that's your Plan B -- just get your money back, even if you forego the functional outcomes of a successful Plan A.
But you can't go through life buying insurance for every risk. At some point, you need to ask the 'affordability' question. If affordable, don't insure it; otherwise, look at mitigations.
Losing function
But functional or relationship loss is altogether different. You can't buy insurance for that one. There really may be no Plan B.
What then?
When there's no Plan B, then you change behaviors
It's all well and good to follow the first rule: Make Plan A successful.
But mitigating a functional or relationship loss is often and largely about personal interactions, behaviors, personal risk taking, and pushing limits.
And only you can sort this; only you can assess the cost to yourself -- not necessarily dollars.
Think about the cost if there's no Plan B!