The time frame for a buyer to complete their due diligence is stated in the purchase agreement. It is up to the seller rather to extend the time or not. I would review the dates with your realtor.
That would require a legal answer and we are not lawyers, you would need to run it past a lawyer who will need to see the survey and what sort of encroachment it is. I have seen some not worth worrying about and others that require legal action.