"The South intended, and was working hard to, make slavery legal in every state in the Union, whether those states wanted slavery or not."
"The South was over-represented in Congress due to slaves being counted as 3/5s a person for assigning representation, but as slaves could not vote, all that political power was concentrated in the hands of white slave holders"
These statements contradict the history of compromise in the attempt of equal status of free and slave states. That the South did not have the political power to make every existing state or new state a slave state, this is easy enough to see. There was no equality between the number of slave states and free states after 1848; in the majority of the years prior, the free states outnumbered the slave states. Again, there were no new slave states admitted to the Union between 1845 to 1863 but there were six free states admitted. Even when Abraham Lincoln admitted West Virgina as the newest slave state there was no equal balance, not even close.
You are correct the slavery economic system was based on land, but only on land that was suitable to profit from that system. That land, by 1850, was already taken, there were no new suitable territories to expand the system to in the United States. This is why in the early 1800s slave states began forbidding the importation of slaves, which would have depreciated their value with over population.
The number of representatives House in the 36th Congress was made up as follows...
North Carolina 8
North Carolina 8
South Carolina 6
Total slave state representation was 85 out of 241. Not really an over powering majority.