1. Yes, you wired the NEMA 14-30 receptacle correctly. That receptacle has two hot connections, each 120V but on different phases, resulting in a 240V potential. So if you take a voltage tester you will find it reads 240V between the two rectangular slots on the outlet, and 120V between either rectangle and the neutral or ground. You can check this using your voltage tester, but it sounds like you did it correctly.

2. Figuring out where that wire went is going to be tricky. If you can't find it visually or by process of elimination, the best tool to help would be a tone & probe kit. You'd hook up the tone generator to your mystery wire (after double-checking that it's not live) and take the probe component with you. When the probe is near the toned wire, it will beep, even if it's a few inches away inside a wall.

You must always follow the most recently adopted code when installing new wiring, no matter how old the house is. With that said, the cord of a cord-and-plug attached device is beyond the scope of the National Electrical Code (NEC).

Installing a new range

When you purchase a range, they will also sell you the attachment plug. If you opt to install the stove yourself, they will ask you if you need 3 or 4 wire. This is because if you already have a 3 wire receptacle that was installed before the code change then you are not required to update the house wiring when installing the range.

When you're ready to install the range in the house, you'll follow the manufacturers installation instruction for a 3 wire attachment plug. Once the cord is attached to the device, simply plug it in and you're done.

Installing or modifying a range circuit

If you are installing a new circuit for the range (which it sounds like you might be), or modifying the existing range circuit, you must follow all currently adopted codes. This will likely mean running 4 conductor wire, and installing a 4 wire receptacle. Remember, the code is applied during installation, not when the materials are purchased.

Side Note: Technically, code applies at the time of inspection. However, the inspector will usually use the date the permit was granted. So if there was a code change after the permit was pulled, but before the inspection, the inspector will usually use the old code.

If your range has a 3 wire attachment plug, you'll have to follow the manufacturers installation instructions to change to a 4 wire plug.