Yes, all devices which are able to connect to the internet, can do so via powerlan as well.
Powerlan uses the electricity grid to transfer network data. Every socket using an adapter connects with the electricity grid. If you connect your router with an adapter, you can use the internet as well.
Yes, if you buy more of the same adapters, and connect them to your network. You're advised to use adapters of the same brand and type for the best connectivity.
This can have the following causes:
1) It could be that your electric wiring is damaged or of a bad quality.
2) Electrical devices on the same circuit can interfere the signal on that circuit. Like a microwave, air conditioning, hairdryer etc.
3) If there are (a lot of) fuses between two adapters, the signal can be weakened.
4) If you use different types of adapters, the signal's quality can be affected. If each of the adapters are connected to a different fase of the electricity grip, the signal can be interfered. Connect the adapters on the same fase of the electricity grid.
Cable internet still is the fastest data connection between computers. However, cable internet uses separate cables to connect computers while powerlan uses existing electricity cables tot connect computers, thus saving the cost and effort of setting up cable connections.
If the adapters are connected to different fases, interference can occur. Ensure that the adapters are connected to the same fase. In case you use several different fases, ensure that the router is connect to each fase separately by using multiple adapters.
Powerlan is a solution if you want to have access to the internet in places in the house which are difficult to reach, like an attic. You don't need to set up cables, all you need is a socket connected to the electricity grid.