@George3G 

Hi,

It depends what you mean by "resolve these two issues". 
Truly resolving the issues would probably require installing a few Wi-Fi access points that are connected to the main router using Ethernet cables.
By using Powerline adapters you are not going to get 300 Mbps throughout the house.

If you want to stick with the Powerline adapters, then bring the two units as close together as possible. Use longer Ethernet cables to connect them if necessary.
If you currently get a third of 300 Mbps through the Powerline connection, that would be 100 Mbps. With a bit of luck you might get up to 150 Mbps after moving the Powerline adapters to other sockets that are closer to each other.

As for the Wi-Fi of the Powerline adapter. I doubt it has been designed to cover a large area with strong Wi-Fi signal.

And that upstairs/downstairs thing is always problematic, because the antennas of most Wi-Fi equipment are designed to transmit their signals mainly in horizontal direction, not vertically.