Yes, it is legal in the sense that it cannot be stopped and the votes would be valid. Some Electors may incur (Constitutionally dubious) fines or penalties by not voting as pledged.
No, it won’t make a difference unless a large number of Trump / Pence Electors defect.
The only thing that could possibly cause that would be something that utterly discredited/disqualified Trump, to the extent that if he were elected he’d still face likely impeachment and removal. And that would basically send to the House Clinton, Trump and whatever Republican got the most of the dissenting votes — Pence? Romney? Who knows.
I am not sure that laws requiring electors to vote for the winner of the electionin that state are questionable Constitutionally at all. The only thing the Constitution has to say about choosing electors is that the individual state legislatures have the power to determine how the state’s electors are selected. It places no restrictions on laws regarding how electors are to vote.