1. Once colonizing Europeans established the plantation system on the west side of the Atlantic, what labor sources might they have drawn upon instead of enslaved Africans? 2. Once the Portuguese found a way to navigate down the west coast of Africa, was it inevitable that a slave trade would begin? To what extent was the slave trade forced upon West Africans? What were some of the dynamics of African complicity in it?

Often when we think of the Atlantic Slave Trade, we assume that all plantation labor in the Americas was performed by African slaves, and in many cases, that is true. However, some plantations in the New World, or Western side of the Atlantic, were operated by the indigenous Americans that lived there prior to European colonization.
Once the Spanish learned of the New World and its inhabitants, the Spanish crown was eager to colonize and profit off of the rich and fertile lands of the Americas. One system they developed in order to encourage Spaniards to migrate and set up a profitable Spanish plantation (which would benefit the crown due to the mercantile economic system) was called the encomienda system. Please check out the picture I've attached for a fun visual!
The idea behind the encomienda system was to grant plots of land and indigenous inhabitants to middle-class Spanish families and/or merchants to encourage them to colonize regions in the New World. In exchange for profits, the Spanish crown offered them a new start and the potential for social mobility - by owning land and laborers, they were better off in the New World than they were in Spain. So other than African slavery, we know that indigenous laborers who survived the initial exchange of European diseases (the most well known would be the Great Dying pandemic of smallpox which killed an estimated 50-80% of indigenous populations in the Americas).
More on the slave trade. Once the Portuguese made their way down the coast of Africa, it was likely that slave trade would occur. This is because slave trading was a process already present in Africa and was not contingent on the Europeans. Prior to Portuguese, Spanish, English, French, and Dutch slave trading with strong African coastal kingdoms, slave trading had existed in Africa as long as Africa's tradition of human civilizations. Not only can we trace slave trading back as far as ancient Egypt, but we also see extensive slave trading between the Post-classical/Medieval Kingdoms (like Mali, with its most famous leader Mansa Musa, often hailed the richest man who ever lived) and Muslim merchants in the North. In fact, slave trading continues today in Northern Africa (check out the article cited below if you are interested!)
Once the Europeans found success in the New World, however, is when the slave trade was escalated. That was contingent on European trade. As plantation agriculture became immensely profitable (primarily with sugar, secondarily with cotton, tobacco, indigo, rice and coffee), Europeans built even more plantations that required even more slaves. In addition, the conditions on sugar plantations in Brazil and in the Caribbean were so harsh that life expectancy was low and plantation owners needed to continually replenish their supply of slaves. All of these reasons escalated the slave trade, which completely changed trading interactions between African kingdoms and European merchants and nations.
https://af.reuters.com/article/topNews/idAFKCN1IV1R5-OZATP