Is a P2WPKH address synonymous with a Bech32 address? BIP142 when encoding a segwit P2WPKH address is using a base58 address format (address p2xtZoXeX5X8BP8JfFhQK2nD3emtjch7UeFm in the example). 

I understand that BIP142 predates BIP173 but are P2WPKH and Bech32 synonyms now? Is there such a thing as Base58 P2WPKH address?
are P2WPKH and Bech32 synonyms now?


No. Bech32 is also used for P2WSH addresses. Native segwit and Bech32 are synonyms.


  Is there such a thing as Base58 P2WPKH address?


No.