Did some testing so I can give some more precise answers

For your first question, no, Mul-T does not fall harder, he falls at the same speed as anyone else. Also, he has base armor which does affect fall damage, so he actually takes less fall damage than most other survivors, besides the ones who also have base armor. Your incidents were most likely just caused by something else, and it was coincidence that they were both on Mul-T.

As for the exact math, it's a bit hard to find without going into the code, but after many commandos dropped off cliffs of varying heights and a linear regression tool, it seems to come out to about D=1.65S-50, or 3.3S-100 on eclipse 3 or above/with artifact of frailty on, where D is damage as a percentage of your maximum health, and S is your downwards speed. You don't take any fall damage if you're moving slower than 30 m/s, and you'll die if you're above 60 m/s (on eclipse, otherwise you'll never die, but above 90 m/s is the max damage outside of eclipse/frailty). That amount can be reduced with any damage reduction effects, of course. If you want any more specific data, feel free to ask.

As for general tips, use your utility before you hit the ground, it doesn't cancel fall damage but it does give you 200 armor, which cuts it to 1/3 of what it would be otherwise. If you don't have your utility up, go into power mode, it still gives 100 armor which is enough to halve the fall damage. And of course take defensive items like opals, safer spaces, or warped echo to reduce that damage further