P2SH-P2WPKH private key or revert to P2WPKH Is there a way to get the private key out of an P2SH-P2WPKH address, using Bitcoin Core 0.15.1?
If not, is there a way to get the origin P2WPKH address to get the private key out of this address?
In 0.15.1 you'll need to find the P2PKH address corresponding to the key (it's the argument to addwitnessaddress you used for creating the address in the first place), and pass that to dumpprivkey.

In 0.16, things will likely just work.