What if the Spaniards were able to keep Portugal in the Iberian union? How would this affect the Atlantic Slave trade? How would American colonies develop without slaves or with less slaves?
You seem to be asking two unrelated questions. How would the Iberian Union cause a drop in the Slave Trade? You have a demand for slaves in the colonies, so the supply will be met somehow. I wouldn't expect too much difference from OTL.
Well I think a longer lasting Iberian Union would have an important impact on the Atlantic slave economy as while the total volume of Slaves being moved would be similar Britain would be much poorer. The Portuguese largely supplied their colonies with slaves in their own ships from Angola, but the Spanish were heavily dependent on British Slave Traders. With an Iberian Union I suspect it would be Portuguese rather than British ships supplying the Spanish colonies. So you have a poorer Britain, with a less developed maritime economy and that probably means a poorer and less populous British North America.