Not to mention that Congress was held by a Southern coalition until 1858, not a Northeastern one.
No quite the case. In the 1840 election for House of Representatives, slave holding states elected 87 Representatives. The total number of Representatives in 1840 House was 242. The balance of slave vs free Representatives was 87 to 155. Slave holding states did not control Congress. They had parity in the Senate, as long as for each free state admitted, a slave holding state was admitted. Until 1858 that balance in the Senate was maintained.