Powerline adapters tend to have issues with many extension cords not sure why exactly. You would think if I use heavy duty ones that have wire the same size as the wires in the wall it would cause no problems but it sometimes does. Then again you can move the powerline unit from outlet to outlet in the same room and it work differently.
I would avoid any kind of extension cords. If you must try them use the commercial duty ones used for tools, you want at least 14 gauge wire (or less the lower the number the heavier).
Current protection is a simple circuit breaker. Most power strips have these as a little button that pops out if you overload them. Still since it is not recommended to use any extension cord I would get ones...