Casting a vote is when you choose a candidate and give them your vote in a poll/election.
To abstain.
Generally, yes..... but it will depend on the rules / standing orders of the particular committee or organisation.
If someone does not vote then they are not added to the poles and you have no say. If another person votes then that is there say and there rights.
The electoral vote is the one that determines who becomes president, so that one probably matters more. I personally feel that the winner of the popular vote is always the one who should become...