Does home plug use the line conductor and neutral as a pair?
Short answer, YES. The Homeplug devices use the Active and Neutral as a PHY layer medium. In the latest variant (Homeplug AV2) the line and earth can be used as a second pair to increase performance.
Can a Homeplug device send Ethernet down mains wiring?
Short Answer, NO. The Homplug device packages data received from an Ethernet port (the one on the device) into its own DLC protocol packets. It is NOT Ethernet over mains cable.
Your thinking of the problem is flawed, you need to consider the networking stack in totality to make any sense of the schema.
"Ethernet" is used in a consumer sense as a generic term, but there are a slew of standards involved. To start to break this out you need to consider the OSI stack:
Ethernet applies at the lowest 2 levels of the stack:
- At the DLC layer the standard defines a way to packetize data with an identifier for both the source and the destination points in a physically connected network. This has nothing to do with the physical network definition, for example the data could be sent over Coax using 10Base2 or Fiber using 1000BaseLX10 or over Cat 5 cable (probably what you consider to be "Ethernet") using 1000BaseT.
- At the Physical layer (PHY) the DLC frame of data is encoded for whatever means of transport is being used. This is a good explanation of the "Ethernet" PHY's
So to get to your misunderstanding. The Homeplug device DOES NOT in ANY WAY send Ethernet over the mains. The devices DOES present an Ethernet port, but the data from this port is split down to the basic layer two packet and re-encoded to be transmitted over the mains cable.
The modulation (or PHY) interface used by Homeplug devices has ABSOLUTELY NOTHING TO DO with Ethernet signaling, so don't think of it that way.
If you want to understand the HomePNA PHY interface you could start here.
As a hint to how to think of these networks of networks try to learn about MAC addresses (DLC, layer 2). In a Homeplug network the multiple devices must be able to distinguish (or be paired to create a virtual network) between endpoints. Homeplug take the pairing route so becomes a simple point to point network, but you could create a multipoint network under the the same HomePNA specifications.
As a really high level example, you could connect from your computer to a remote laptop computer on another continent. Your computer talks to a router/cable modem using an Ethernet port ….the remote computer is using WiFi. The two don't match, but you can talk. What happens in-between your computer and the laptop is unimportant and certainly is not Ethernet over anything. It's about protocol handling at OSI layer 2 and below.
If you want to understand more about the Homeplug networking architecture, read this introduction.