Asking the Embassy to reschedule is the most responsible thing to do. You have two issues to overcome. You might go into the interview and come away with just a 221g to get those two documents or you could get a denial.
The Embassy might understand about the I-134 because of the volcano but then again, you are supposed to have all papers in hand before returning the chcklist and requesting an interview date.
As far as the Police Certificate, you do not have as good an excuse as a volcano.
I would ask for a postponement.
