But Wayland isn't a thing on its own, there's no "Wayland server" or anything else equivalent to the X server. The compositors like Kwin or GNOME's Mutter are Wayland implementations fully responsible for handling the display output.
You can blame Wayland for the lack of universally supported global hotkeys or for issues with apps that need to know exactly where on the screen they are - these are issues with the protocol - but not for bugs in one compositor's implementation of display management.
Yes, but these internal connections can be done in a variety of ways - for example the most common way to connect laptop displays (which I would definitely classify as internal) is using embedded DisplayPort.