Think of it as using the ChromeOS kernel (which suits the hardware configuration perfectly) and then running the Ubuntu userspace (which co-exists with the Chrome OS userspace).

Even if you don't use the Chrome OS userspace much, removing it won't save you enough to make it worth it.

Yes, there are plenty of howtos on how to do this. The only downside is you have to enable developer mode, so you lose trusted boot.

It depends a bit on the model, but in general yes you can. I haven't done it on a Chromebook, but I've completely wiped out ChromeOS and ran Arch on an Asus Chromebox (using instructions meant for the very similar C300 Chromebook). Worked perfectly, great little machine. Only hiccup was I had to flash a custom bios.

Yeah, you can. Usually you unscrew the laptop, remove a screw on the motherboard and you get SeaBIOS and you can install any distro you want.