Question

How are a mix of pre-1982 and post-1982 pennies a misleading or incorrect analogy for actual element isotopes?

Answer 1

Because the old pennies were 95% Cu and 5% Zn. The new pennies are mostly Zn with a small amount of Cu; therefore, the mass of the old and new pennies are different. That would prevent you from using the masses and averaging them. You can read more about the old and new compositions here.
http://en.wikipedia.org/wiki/Penny_%28United_States_coin%29