I don't really know that you can extend settlement on the basis of the purchaser lying to you. You could simply ask (via your conveyancer) if an extended settlement would be suitable.
Have you exchanged contracts? If not, then the settlement period can still be negotiated. If you have exchanged, then you can mutually agree to change the settlement date, but if they don't agree then you're probably stuck with what is on the contract (which begs the question, why did you agree to it?)
As for the mail, big deal, but you've got about 4 choices.
1. Forward the mail to the purchaser.
2. Put the mail in a box and leave it behind when you leave (probably what I'd do because it's the easiest).
3. Mark return to sender and post it back.
4. Throw it in the bin (I think this is illegal, but nobody ever seems to care these days; this is also a bit rude on your part).
Regardless, I don't think it's worth getting upset about. Given they appear to be new immigrants, they may not have anywhere else to send it to.