Polk was pro-slavery, more so than Clay, so I would think that abolitionists would support Clay over Polk. However, enough voted for Birney to keep Clay from winning.
I don’t think any abolitionist president was elected between JQ Adams in 1824 and Lincoln in 1860 (with the possible exception of Van Buren in 1836). It wasn’t exactly a time of principled national politicians.
2) True for the same reason as above. See my opening chapter in the new book out Tuesday, "Martin Van Buren has a Nightmare and Creates the Political Parties . . . in the 1820s," in Seven Events that Made America America. VB wanted to crush all discussion, pro or con, about slavery and did so with patronage or bribes. Sounds just like Sestak.