Since the 1983+ pennies are so inherently weak, are coins like this 2020 penny, (reverse) considered errors or just the expected status quo?
Copper plated Zinc Cents started mid 1982. They are prone to all kinds of environmental damage that will affect the plating.
No, but you need some experience to know what is damage and what are errors. In the case of your 2000 above you have a case of misaligned dies. In this case both dies appear to be misaligned. The obv toward 8:00 and the rev toward 6:00. The discoloration of the reverse is just that, staining that occured after it left the mint.