Is the scalar wave phenomenon real, and if so, why is it not much in practical use?
About half a minute of googling solidly tosses the concept into the bunkum bin.

Here's a link on those "superluminial waves" - explained quite succinctly as a trick of perspective combined with relativistic speeds.

And here's a not quite as kind link on your "scalar waves" - complete hooey, it looks like.

I don't really see how any of this is related to EE except tangentially through the idea of waves, though.

EDIT: To address your update, scalar fields are an actual thing in physics. "Scalar waves" are not.