Qavion:
In the descent with tailwind scenario you posted, VNAV will allow up to 15 KIAS above the bug speed, but the bug will stay in the same spot (controlled by the setting in the VNAV Descent page). Overspeed protection is still active. If the winds diminish, the airplane will return to the original commanded speed after the VNAV path is regained.
If VNAV Path cannot be followed at +15 KIAS, options are: speed intervene, reset VNAV Descent speed, go to FLCH at a higher speed, deploy speedbrakes, extend landing gear and/or flaps...