It's impossible to say without seeing the purchase and sale agreement, as well as the escrow agreement (if any) with the title company. I would start by trying to figure out who is at fault for the delay. What does the escrow agreement say about the processing time for cashiers checks?
Then take a look at the purchase and sale agreement. It should address what happens if closing does not occur on the closing date and should set forth the parties remedies.
As far as the prorations, if you don't have the HUD-1 yet, it doesn't sound like they've actually been calculated. Barring something to the contrary in the purchase and sale agreement, it would be reasonable to calculate the prorations as of the actual closing date, instead of the date set forth in the agreement. The actual closing date will be the date that money is released to the seller from escrow and posession is transferred to the buyer.