[Zeke]

Bill's tester is not faulty. He sees 115v where it is supposed to be and he sees 50v where he should see 115v. And the resistance he sees is exactly why. It's Ohms Law.

That black wire is now useless. 4 wires is enough to have 2 operating circuits to the garage. Sure, if you took the yellow and the "new" hot and measured across them, you would see 230v as long as they are on different legs. The fact that they are on one side of the breaker box suggests they are. And they certainly are if they are next to each other.

Only one neutral needed for those and a ground for safety. It is a misnomer to call a neutral a ground. A ground is an equipment ground and safely grounds the whole system. The neutral completes the circuit by connecting the loop back to the box. The fact that they terminate at the same point does not mean they come from the same point when something is energized by that circuit.