My goodness me, InChina - what fun you must be having
I assume your CP will soon be an F/O?
A few observations which may help you:
Cost Index is relevant - VNAV path will 'attempt' to fly the appropriate speed for CI30 in descent as a target.
I spent many years on 3/4/5/700 filling all the boxes on the descent page, including inside leg, collar size, date of birth, to find it STILL screwed it up
I gave up and put nothing in the box UNLESS there was a tailwind for the descent to avoid the T of D being calculated 'too late'. IF there was a tailwind, that amplified your problem putting you much closer to Mmo.
I would recommend to you, if this 'culture' of doing it 'the Captain's way' exists as you say, that you politely decline if you are unhappy - no major issus - you may be thought of by Cpt X as a 'chicken' or whatever - so what?
If you are in a position where you have to do it in VNAV, I always found LVL CHG with a sensible speed entry was a better option down to below 30k. Generally except for howling tailwinds or really late descents, the 'drag required' can be ignored (provided the speed is controlled) and the path picked up later in descent where there is a greater speed margin as said above - all this, of course, subject to any fixed altitude constraints
Have in your mind where it SHOULD descend for the expected wind conditions and chosen speed etc, and if
it does not agree, fly it your way.
Good luck!