1. What is Johnson's stance on communism in the TL?
Much as it was in OTL, actually. With the fall of Vietnam, the Populists are perceived as "soft on Communism", so any Populist president is going to feel pressure to take a more hardline stance against the Soviet Union in the future.
2. Are any People's Party candidates calling for more environmental regulations?
Welsh, Humphrey, and
possibly Johnson could probably be persuaded to support new regulations if such a bill passed Congress. That said, the People's Party is largely reliant on the farmers in the South and West who form their electoral base, so it will be hard to get a Populist to support expansive reform. To be honest, if environmentalism is your main issue, Scranton is probably your best bet.