The rust inside is irrelevant and has no effect on anything

I don’t have one, for the reasons I gave. Adding complexity without getting any benefit from it is never a good idea. This goes for any application really.

A regular hydraulic floor jack will last you for many years without ever giving a problem. Mine gets beat to hell, left out in all weather at times, etc, and never so much as complains, and if it does it likely only needs a little top up of oil. It only takes 30 seconds to jack up the car with one, so what exactly are you saving by getting a powered one? And then when something does go wrong, other need replacing or take 10x as long to fix.

Neither. Buy a normal hydraulic floor jack and a decent set of jack stands. Unless you are jacking up 5 cars a day, having a powered jack is just a waste of money and more things that can go wrong.

What now? You will get major degradation within three months even in a sealed tank. If you’re going to store fuel for longer time, you need to use a fuel stabiliser to prevent it going stale.

If you are touching the terminal to anything with paint on it, you won’t get an accurate reading.

I think you answered your own question? You have a leak. That isn’t a master cylinder problem when the leak is at one of the wheels… Find out where the leak is and rectify.