For some reason as you stated the brakes were upside down on the fronts
This is a good thing! It means you don't have to spend $300 on a new booster chasing down a problem that didn't really exist.
So am I understanding your correctly? With a semi firm/ firm pedal when the car is off... can go to nothing even when the bake booster is working?
Thats what the booster does, it helps you brake. When the car is off, your foot has to do all the work. With the car on, the booster puts pressure on the MC. If you aren't completely bled, then the booster will compress the MC and make it seem like you have no pedal.
My old Isuzu's brake pedal would drop about an inch when you turned the car on. Same deal here, except that system was bled so after the drop the pedal was firm.
Also, for anyone looking at this in the future, the failure mode for the booster is a lack of vacuum, and never 'too much.'
I just watched your video, and you can see the brake fluid decreasing (ALOT) inside of the reservoir with each pump of the pedal. I would do your bleeding and then monitor that fluid level VERY closely, to make sure that there is no fluid ingress into the booster. It could be a function of not being correctly bled, but in the future if you ever LOSE vacuum, its possible that fluid has bypassed the MC, been dumped into the reservoir, and sucked in through the check valve and burned. This happened to me just recently. The fix is a new MC.