I see a pattern emerging
As for your problem, I would suspect that if there is enough of a connection for get power, there should be enough of a connection to powerline ethernet.
How is your house wired and how new is it? for example, do you have a ring main for upstairs and a seperate ring main for down? This is a hige impact of poweline ethernet, as it has to traverse the fuse box/circuit breaker. This is something that isn't recomended for PLE.
Having multiple connections (wireless and ethernet) won't stop you from being disconnected from things like Messenger, Skype, games etc. as you will end up with two seperate IPs (one for each connection) and traffic will not route between them. You'll have to establish a new connection when the one that app is using goes down. This will probably just mean signing in again, but it could be a pain. Will halt downloads too if whichever connection is being used drops.