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.