Nothing is being charged back. I bought 12.5 BTC for $70, but only recieved 4.5 BTC. Therefore I am owed 8 BTC. He does not owe me any USD at all.
I'm fine with just giving R- a scammer tag and leaving it at that, unless he wants to send the 8 BTC to me and also the 36 BTC owed to OP. However, I think that outcome is unlikely.
You won't lose your PayPal account if you call your credit card company and ask for a chargeback because of fraud. It was fraud on the part of the seller as they did not give you the items you paid for. I have done chargebacks for thousands of dollars (because of hosting companies failing to live up to their TOS) and my PayPal account has never been closed.
It takes about 30 - 45 days, but the longer you wait the more likely you won't get your money back.