Jose wrote:
So, how far over redline is it "ok" to operate an aircraft, and how
arbitrary is that red line (both on the tach and the airspeed indicator)
and how is it determined?

Red line airspeed is a pretty specific thing. Unlike yellow line which
is only a problem in turb, red line is the point that things (like
wheel paints) begin to take damage. Red line is not a "if lots of
things happen it will be a problem" its a "something wasn't made strong
enough to do this".