A PE of 5 on a company that would grow or contract 0% for the next 100 years would be able to pay a 20% dividend for the next 100 years.

It completely depends on the expected rate of growth of the company. Even 100x can be fair for a very high growth company. For companies that don't have such great prospects, <5 may even be appropriate.

"PE should be close to X or between A and B" was always an extremely rough and imprecise heuristic, and it is no substitute for a real analysis with a DCF model.

Depends on the interest rates and bond yields. 20PE is like 5% profit, which is better than most bonds in the past ~10 years if the business is stable. When yields rise it will become closer to 10-15.

5 is definitely an undervalued company unless the business is risky or expected to decline in profits.