1. A sacrificial lamb in this case would be someone or something you know will lose, but must go through the loss or pain anyway. In this case, the Republican party put Goldwater up for election, even though they knew it would probably be a humiliating loss.
In general, a 'sacrificial lamb' is somebody that must go through some pain or loss for the good of a larger number.
2. In the electoral college, each state has a given number of votes. If a majority in that state vote for candidate A, then A gets all the state's votes - he/she has carried (won) the state.
3. You are correct. They could estimate it, but did not know for sure.