Then your vote is diluted and carries less power to keep someone off, since many of the choices have no chance of winning. If you select only the most popular who are not bad your vote stayed focused.
No, that's incorrect. Read the voting method before you make such statements. Simply speaking, the number of votes for each candidate is counted. Suppose you want to get A in and keep B out. If you vote just for A, A gets 1 vote from you, and B gets 0. If you vote for everyone but B, it's exactly the same.
Exactly. It doesn't make any difference how many people you vote for, just who you vote for. Basically, you vote either Yes or No on each candidate. It's possible that voting for extra people will prevent your favourite candidate from winning, but voting for extra people can never cause a candidate you didn't vote for to win.