Whomever gave you your oral needs to be cut off at the knees. There is no reason that an examiner should be asking you about pressurization if you're not flying a pressurized aircraft, or about the gulf of mexico, or about flying in Alaska. Why a "contactor" for a master and not a switch? Most systems use a switch, typically with a relay, which enables a small voltage and amperage to control a larger one...but the simply answer is "that's how the manufacturer designed it." You're being asked too much, and being asked to answer questions that should be part of a commercial oral. The commercial is a glorified private with a couple ore maneuvers. Someone is overthinking it...but you need a different examiner. Whomever did your oral should have his designation yanked.

Why doesn't your car use magnetos? Because it's not in the stone ages, and it uses electronic advance for a more efficient, smoother engine operation, longer engine life, and better fuel economy and power. Why does your airplane use magnetos? Mostly because the certification process is so arduous and expensive, and so bureaucratically challenged and the market small enough, that getting equipment and systems certified is too expensive. We use magnetos because they keep on chugging when the electrical system dies, and in theory are more reliable but the experimental world makes excellent use of hybrid systems or even straight electronic ignition...and turbine aircraft don't use magnetos. Piston airplanes do because even though they're troublesome and failure-prone and very limited in their ability to provide a spark (single timing), they're supposed to provide redundancy, and they're more of a functional leftover relic from a hundred years ago.

You should absolutely understand unuseable fuel. It's a critical concept, as is fuel in general.

What on earth was the examiner doing grilling you on oxygen systems? What kind of airplane are you training in?

The ASA oral prep guide should be more than enough and representative of the type of questions that should be asked. Don't go back to this examiner. You'll be much better served by another.