You are right they dont in most cases, but i myself have managed cases where removal of a plate was advised , this was a case where removing a spinal tumor was complicated. The reason was essentially the same that i mentioned above. Without a MRI removing the tumor was difficult, so they first removed the plate in a minimally invasive manner without disturbing the tumor, the tumor in this case was slightly below the plate and then did a MRI.
In your case, they may remove the plate without going from the side of the tumor so as to prevent any issues with the tumor, it is possible that their plan is to remove the plate by entering from the other side of the heel.
Yes a biopsy can be done initially, that would be done under ultrasound guidance or even blind if need be, that will tell us about the type of the tumor but NOT its extension. If it is a benign tumor then yes it is less likely that it would have invaded into the surrounding tissue and may be removed without a MRI but if it is a malignant tumor or aggressive tumor then a MRI would be needed.
To answer your last question, YES they can do a biopsy on a tumor which is present ON THE SURFACE but not deeper biopsies which need MRI guidance. Surface biopsies can be done blindly without MRI guidance and would not need plate removal. DEEPER biopsies would need MRI guidance and warrant plate removal.
In your case IF the tumor is ON the surface, then a biopsy would not need plate removal, the surgery on teh other hand may need it.