If half of the Republicans vote for Boehner for Speaker, half of the Republicans vote for an alternative Speaker, and all of the Democrats vote for Pelosi for Speaker, will Pelosi be elected Speaker with a plurality?
No. Has to be a majority. But she would be no worse.
No. In the example described, Boehner would get 123 votes, Republican candidate X another 123 votes and Pelosi 188 votes. She would have a plurality, of course, but to be elected Speaker, a candidate must have a majority of the votes cast (not including abstentions).