I have agreed on only one item of the topics under discussion, JS: Using displacement in freefall cannot measure mass/weight without acceleration.
You agreed that a body totally immersed only measures volume. Makes no difference whether it is in free fall or not.
If you think about the procedure for obtaining fat percentage (measuring body density) I'm sure you will reason out the necessity of obtaining body volume. You may even come to agree that using a tank of water to measure a person's weight is overkill. I'm not involved in that trade, but I will bet you a week of posting privileges that immersion is used to obtain body volume, and that if they didn't need volume, they could simply use a weight scale.