The Scott Decision did not rest on the Full Faith and Credit Clause. Jurisprudence under the Clause is somewhat complex, but suffice it to say the Clause has never been used to apply one state’s statutory law in another state. More importantly, the Clause does not apply if its application would conflict with a fundamental public policy of another state. A state’s decision not to consider human beings as chattel property would seem to be pretty fundamental.
I don't think Article IV section 2 gives them any choice. If they free the slave, they violate Article IV section 2. If they don't free the slave, then they are defacto regarding the slave as chattel property.
This also begs for the question of how any state law in conflict with the constitution can be a valid law?
That is my understanding as well. The reason the FF&C clause was not used was because instead, Taney argued that neither the feds nor any state could prohibit slavery. That was grossly inaccurate. Until the 13th -15th Amendments, slavery was an issue of each sovereign state.
I think FF&C probably would have been effective in this case as even just recently, Congress had to pass DOMA to prevent FF&C from operating in gay marriage issues between states.