Also when everyone talks about return current, are they talking about the current flowing underneath the trace itself or the plane beneath the trace?(because skin effect makes a trace a 3d structure, can return current flow below on the same trace?)
I assume you're talking about microstrip line, so there is only one ground plane, "below" the trace. (Not stripline where there will be ground planes both above and below the trace)
The signal current flows in the trace and the return current flows in the ground plane.
To the extent that skin effect plays a role, most of the signal current flows on the surface of the trace closest to the ground plane. And the return current flows on the upper surface of the ground plane near-ish to the trace.
Does not via stitching causing the solid ground plane to become broken? Is this okay?
Stitching mostly makes sure that all of your ground planes are at the same potential. It can prevent a semi-isolated region of ground plane from acting as an antenna and producing radiation.
It is a good idea to keep stitching vias at least a few trace widths away from any high speed microstrip traces. Then they will have little effect on the return currents associated with the microstrip.