The chaos and instability which interrupted inter-regional trade certainly delayed the weevil's progress, if it crossed the border two decades earlier it starts to become a problem in the South much sooner, and a more stable Mexico allows this.
I'm not sure how the Confederacy managing independence makes for a more stable Mexico. For that matter, an independent Confederacy is likely to be less stable and less prosperous than OTL's postbellum south. Most importantly, I don't see how this has any effect on the spread of the boll weevil - its life cycle consisted of eating cotton buds in the spring and summer and lying dormant in or near the fields in fall and winter. Unless Mexico is shipping unpicked unprocessed cotton or dirt from their cotton fields to the Confederacy, trade will have no effect on the speed at which the boll weevil spreads.
Why would it remain the same?
I am not assuming that rates of cotton consumption or British investment would be
identical to OTL, I am asking why they would be
significantly different. Bringing Egyptian and Indian cotton fields up to the needed production levels would require the investment of significant amounts of time and money. In OTL, the end of the Civil War and the blockade of southern cotton resulted in the collapse of both Egyptian and Indian cotton markets. Shipping cotton from India to Europe would also take longer and cost more than shipping from the American south. American was also a different species than the cotton grown in Egypt or India, with American cotton being higher quality and more durable than Egyptian or Indian cotton. Distaste for slavery did not make anyone willing to pay more more money for inferior cotton before the ACW, it's unlikely to have a
significant difference on the amount of cotton purchased from an independent Confederacy.
Because then you can't sell them or use them as collateral as effectively, and this hurts small time slave owners as much as large ones. Cheap slaves are no good.
You're only looking at this from the seller's point of view, but every sale also required a buyer. Buyers wanted cheap slaves, sellers wanted expensive slaves. High slave prices made for a greater profit per slave when selling them, but also made it less likely that anyone would buy those slaves. The majority of the slaves sold were in the Upper South, where the prices were lower. The majority of those slaves were purchased by the Deep South, where the prices were higher.
Slaves were also not just a commodity, but a means of production. Putting up one slave as $1200 collateral risked less than putting up three slaves at $400 each. On the other hand, three slaves would produce three times as much labor, making it easier to pay off the loan.
Key word is appears, and this is only after a very significant spike in roughly 1838, which seems to be abnormal. Then the prices steadily rise throughout the 1840s-50s, which is two decades of growth for the internal slave economy and the entrenchment of the planter elite, who owned the majority of the slaves. The drop in prices would still be bad for them and not encouraging ti growth in the slave ownership since slaves would not be as valuable as collateral and signs of wealth.
The drop in prices would only be bad for large slaveholders that were planning on selling their slaves or putting them up as collateral on loans. That's a minority of the large slaveholders and would consist mostly of those who were having financial difficulties. The majority of large slaveholders would not be affected by the price drop.
Owning slaves would still be seen as a sign of wealth unless the price dropped so catastrophically that the vast majority of the population could afford to own slave. And slaves were not just a commodity, they were a means of production. Lower prices would lead to more smallholders and non-slaveholders purchasing slaves to increase the productivity of their farms and businesses.
The expulsion of unclaimed slaves is unlikely since slave owners who would protest a slave being accidentally freed (or would make a false claim which would then be difficult for the government to disprove) would lead to court cases where the government would then have to pay the collateral off on the accidentally freed slave. Simply buying them then renting them out as government owned slaves would be the far more likely option.
The only slaves not purchased by members of the public would be those too ill, weak, sickly, or injured to work. Even some of those would be purchased by free blacks wanting to help enslaved relatives and friends. If the Confederate government purchased these excess slaves they would be spending public money in order to increase public debt, with little to no chance of any return in their investment.
If the Union allows such sales to go ahead, which is either unlikely, or unprofitable. These are going to be areas saturated with escaped slaves and I have a difficult time believing that there is going to a great acceptance of such economic transactions in the Union, if slavery isn't phased out immediately post war, or laws against the sale of slaves passed.
US law prohibited the importation of slaves, not the exportation. The postwar Union probably will end slavery. It is precisely this that will lead to some Union slaveholders selling their slaves to Confederates before uncompensated or undercompensated emancipation can occur. The majority of the general public didn't care what happened to the blacks and some would have actively supported removal of the slaves due to fear of competition for jobs.