Very good question and one that I can't answer. However I would check to see if your router supports QoS. If it does you might be able to ger the router to prioritize important data such as video playback, gaming, video calls etc. I've done that in my router and it made it so much easier to live with 10 Mbit.
That depends on the setup. I use a VPN 24/7, but it's configured on the router so most of my devices are routed through the VPN at all times while at home without them having to run a VPN client themselves, and can also take advantage of DNS-based ad blocking. But yes, it's not a setup for everyone.
What do you mean doesn't work for mobile devices "at all"? It works in many ways. If you have a VPN client on your phone, just disable it while you're connected to your home wifi.