It's tough to see how even a conservative jurist could reconcile DOMA with the Full Faith and Credit Clause. Wouldn't surprise me to see Roberts piss off conservatives again if it his the Supreme Court.
It's very easy to reconcile DOMA with the Full Faith and Credit Clause. By your mistaken interpretation if back in 1855 a South Carolinian had moved to Minnesota with his slaves, those slaves would remain slaves. But the Supreme Court has not been that dumb. It does not interpret the clause in a manner that lets the laws of one State override those of another. What the clause means with respect to marriage is that if you were to visit Myrtle Beach, South Carolina and get married, in a marriage that would be recognized in Minnesota, then when you returned back to Minnesota, you wouldn't need to get a second marriage license from Minnesota for Minnesota to recognize your union.
Any Constitutional overturn of DOMA would have to be based on an Equal Protection argument, not Full Faith and Credit, and in doing so would have to overturn not only DOMA but also all State bans on same-sex marriage, save perhaps that the "marriage"/"civil union" distinction might be maintained.