Hole by Engine Coolant Cap/Best Cylinder to purchase?

[06YarisRS]

I would start by finding out what that white stuff all over your radiator cap outlet is. That certainly doesn't look like coolant to me. Is it a corrosive substance? I've also never seen a hole in the outlet that leads to your overflow tank. Given what's circulating around in your engine, it's no wonder the P0117 code appeared. Your sensor is likely damaged or contaminated. I'm going to guess that your engine has overheated more than once?

Have you checked your engine oil? What does that look like? Is it milky or frothy?

You can't replace a cylinder per se. If you have a damaged cylinder, you either generally have to rebuild the engine by honing the cylinder, putting in new piston rings etc. It's usually cheaper to replace the engine with a used one.

The ABS code is fairly common on Yari. The connector pins at the wheel speed sensor corrode and contact is lost, setting a code.

I don't know what would be setting off your MAF code.

There are no special requirements for mountain driving in the Yaris.

My advice would be to stop driving the car immediately and get it to a mechanic. I'd be inclined to take it to Toyota at this point.