When done properly, there should be an escrow agent. That entity holds the signed and notarized deed. That entity is obligated to surrender the deed to the new owner when the contract is satisfied. The escrow agent will typically send the signed and notarized deed to the County Recorder, who records and sends the deed to the buyer. 
Signed, sealed, and delivered. The transaction is complete.
Any agreement entered into by the seller, after they signed will have no legal standing because the date they signed the C4D will be older than any transaction after. 
Now, if the title wasn't clear on the date you signed the C4D, then that's on you. That's what a title search and title insurance is for.