The constitution gives each state the right to decide how their electors will be chosen. In the past some were elected by the legislature, but for a long time each state has chosen to have them elected directly. It's possible Pennsylvania does give the legislature some power to override the popular vote, but it seems unusual. Also, I may be wrong, but my understanding is that the president's power of pardon only applies to federal crimes, whereas these are state laws.

The supreme court this year ruled that states have the authority to make it illegal for electors to break their pledge, and to punish them if they do (with jail, fines, whatever). What's less clear, iirc, is whether the state can actually stop them from casting the faithless vote instead of just punishing them after-the-fact.