[Ditto]

So I guess the question that the Northern side has to answer is why they wanted to keep the Union together at all costs.

Short answer --- Survival. They knew that and independent slave republic with a 1000 mile border would not likely remain within those borders for long. Slavery could only remain a profitable endevour if there were ample territory to expand in order to absorb the rapidly growing population of slaves and to replenish the worn-out soil that resulted from intensive cash-crop agriculture. The slave economy was similar to a Ponzi scheme --- it could flourish only with every increasing rates of input in the form of new lands.

OTOH, the question that the south has to answer is why did they want to remain slavers at all costs.

See the above.

[Frumious Bandersnatch]

Very good.  I believe that the south had intentions of including much of the western hemisphere in their new nation.  I also believe that it ultimately wouldn't have worked, and the Civil War would have been fought all over again a generation later.