No. Wayland is a display protocol, not a general-purpose IPC, no matter how allergic to DBus you are.
Only the compositor has access to the composited image therefore is the only program able to make screenshots and full screen sharing. Furthermore, efficient sharing of single application windows is only possible with direct memory access to the buffer which also is something the compositor does/has. The Display protocol is the only sensible place to negotiate access to GPU resources.
No, GPU resources can come from any number of other places. That's literally the purpose of direct rendering. Any applications using GPU compute can produce them without requiring a connection to the display server. There's no reason to put that in Wayland or restrict it to the compositor. There's already a much better protocol for sharing GPU buffers: Pipewire. Dbus is only there to act as the glue between a Wayland compositor and Pipewire.