Hi Daniel,
I did not check your calculation step by step, but it sounds about right. Naturally if you change your remaining distance to the airport you also need to change your angle of descent - the best way to do that is using FL CHG mode. VNAV will have a hard time regaining the path - as it descend steep to get back onto the path, it will overspeed and then pitch up - that is the "wavy" descent that you observe.
VNAV descent will ONLY work well if you fly exactly on the planned path and never get too high - otherwise you need to revert to a more basic mode and do the math in your head (remaining distance x 3.3 is a good approximation).
Viele Grüße in den Hunsrück, Jan