If the free cities are dependent on Latifunda to supply the food needed to feed their large populations, does that mean that getting rid of slavery, and thus the workers for these plantations, would lead to mass societial collapse due to not being able to feed the populations? Is it possible that such an event could/will occur in the wake of Daenerys’ ensuing crusade on her way to Westeros?

I would argue it would lead to mass societal change, not collapse, just as the abolition of slavery in our own history didn’t lead to collapse (despite many predictions from slaveholders that it would).

After abolition, you still have a workforce of farmers, you still have arable land, and you still have a market for cereal crops. The only thing that needs to change is how ownership and profits are distributed.