Yes, and ChromeOS is built from Gentoo. That doesn’t mean much, the end user experience is worlds different.

Yes it is, so theoretically people can just install Arch with KDE and Steam and get a similar setup. However, there is something to be said about a supported OS versus Arch Linux’s community support which tends to have little patience for people who aren’t well informed about the workings of their OS.

Yes. But SteamOS is immutable.