Replace crown after root canal?

If money is
not an issue, replacing the gold crown would be the best idea. A bonded
restorative material would be the best way to fill the root canal access
opening, not mechanically retained amalgam. I wouldn’t worry about having the
old crown removed.

You are right to be concerned about the amalgam leaking.  By DESIGN, the amalgam leaks.  Since amalgam can not bond (to tooth OR the crown), the edges will corrode to fill in the micro gap.  Over time, this corrosion breaks down, allowing bacteria inside.  In your case, to reinfect the root canal system.While the removal of the crown CAN damage the tooth, the amount is usually insignificant (it depends on the aggressive nature of the dentist).  This is a common procedure and should be uneventful.

This is a tough one.  Taking the crown off is not a problem and shouldn't hurt the tooth. If you were my patient, you would be getting a small gold inlay to fill the hole, not a silver/mercury filling that will corrode very quickly and leak.  And I wouldn't be charging for that, especially if you paid me to do the root canal, too.