What "Bake GI" does?
it bakes global illumination. You need to do this after selecting the GIProbe node so that its results are visible.
Can you have more Baked Lightmaps and change which one is shown using code?
Yes. The same goes for GIProbe.
Can you use Baked Lightmaps and GIProbes in the same scene?
Yes, though I don't see any use cases for doing so.
Will GIProbes support ray tracing in realtime in Godot 4.0 using the Vulkan Render?
GIProbes can be fully real-time in Godot 4.0, but this is different from raytracing.