If one kidney fails, can the other survive for another 20 years?

Yes. Absolutely. But if the failed kidney is because of a systemic medical condition, like high blood pressure or diabetes, then both kidneys are affected, and the one left may not be too healthy.
It depends. That is a question that no one can really answer. There are too many variables including your other medical issues, genetics, etc. But with close follow-up, hopefully, it will remain healthy for a long time.