Testnet is basically a working client but not ready for mass use, in this phase some internal tests are conducted followed by public testnet launch when people can join and test Emporium and leave us some feedback about some possible bugs that needs to be fixed before Emporium mainnet blockchain goes live.
EMPC - XEP swap is the procedure when the coins from this POW distribution blockchain will be migrated to the Emporium marketplace blockchain.
Onion layer is an additional anonymity layer using Tor Onion network that will be implemented in a later development phase, every detail will be released once we are coming close to it.