Battery seems completely dead until I twist the negative cable on the battery terminal, then it starts.

Disconnecting the battery can sometimes cause the vehicle's computer to reset, which can result in it entering "limp mode" to protect the engine and transmission. This mode is designed to limit the vehicle's speed and power to prevent further damage until the underlying issue is resolved.
To get your truck out of limp mode, you may need to reset the vehicle's computer by following these steps:

1. Turn off the engine and remove the key from the ignition.
2. Disconnect the battery terminals (both the positive and negative cables).
3. Touch the positive and negative cables together (away from the battery) to discharge any remaining energy in the system.
4. Wait for at least 15 minutes before reconnecting the battery terminals (first the positive and then the negative).
5. Start the engine and see if the limp mode has been resolved.

Fully charged your battery should measure 12.6 volts or higher. So based on you data, your battery is a little weak. Not sure how old it is or what brand, but as the battery ages it weakens. I suggest you first make sure the terminals are freshly cleaned using a terminal cleaner and then take the vehicle it to any parts store and they will load test it for free. Good luck. Hope this helps

First does the battery have good charge (12v). Does the wire running into the battery terminals have complete contact with the terminals? Sometimes the wiring into the terminals fray and or slide out away from the terminal housing. It could be a few things but try those first

Check the cleanliness and tightness of the battery terminals. Loosen, twist and retighten them. Also check the cleanliness and tightness of the black earth cable where it attaches to the engine block/bodywork. A bad earth can cause the symptoms you describe.

A quick check you can do is to connect a jump cable to the battery negative terminal - the earth - and connect the other end of the cable to somewhere suitable on the engine/bodywork.

If the car starts no problem with the jumper cable - it's pointing towards a bad earth/connection somewhere on the battery earth

The battery for your car is located in the trunk. There are jumper terminals located under the hood in the front of your car. There is a Red (+) terminal that is under a little black cover on the passenger side of the engine bay near the firewall. Right near that is a silver/gray bolt that is bolted to the frame of the car. This is the negative terminal.

Make sure both cars are and all accessories are off:
1) Connect the Postive cable to the dead battery
2) Connect the Positive cable to the good (BMW) terminal
3) Connect the Negative cable to the good (BMW) terminal
4) Connect the Negative cable to the dead battery
5) Start the BMW. (if you have 2 people then gently rev the engine)
6) Start the dead car.
7) Let both cars idle for a while before disconnecting any cables
8) Remove in the opposite order.

Your jumper cables are most likely red and black. Locate the positive and negative terminals on your battery, they are marked ( + ) or pos.,that's the red one, and ( - ) or neg. that's the black one. Conect one end of your cables to your battery (doesn't matter which one you do first) make sure you are connecting the other end to the same terminal on the dead battery (that is to say positive to possitive) now do the same with the other cable. There may be a little bit of a spark that's normal when you make the final connection. The danger is in taking them off, do it in the same order that you put them on, if you take both cables off at the dead battery and they touch you are going to be having a bad hair day. the thing that most people don't get is you have to have a good clean convention. If you don't try wiggling the clamp on both batteries to make them dig into the terminals. Also there is a huge difference between someone who left the lights on in parking lot and someone with a dead battery on the side of the interstate. The guy in the parking lot has a dead battery, the guy on the side of the road has a dead battery and a dead alternator, he needs a tow truck, not a jump.

If you disconnect the negative battery cable and connect a 12V test light between the battery negative post and the negative battery cable terminal the light will burn if there's a draw. Make sure all electrical consumers are off and all doors closed when doing this draw test. If the light is on start pulling 1 fuse at a time until the light goes out, when it goes out that's the circuit that has the draw. Trace it from there.

The battery cables need cleaning. Remove each batteyr cable and completely clean the cable clamps and the battery terminals.
Also, check the negative cabe where it connects to the engine ground. It is either loose, corroded or otherwise not making a good ground connection.

Sounds to me like a poor battery cable connection. I am not familiar with the BMW cable routing but it should be simple enough to follow and repair.

First disconnect the negative cable from the battery and leave it disconnected until the final step.

Dicsonnect the positive battery terminal, clean the battery and cable terminal with a wire brush or light sand paper. Apply some dieletric grease or light coat of chasis grease and reconnect the cable to the battery. Follow the positive cable and do the same procedure to all connections down to the starter. Along this route you may notice a smaller wire connected enroute with a fusable link or fuse. Make sure to check these for loosness and or melted wire coating this can be a sign of a loose connection and your problem.

With the positive cable connections completed and reinstalled. Do the same to the negative cable, remembering not to reconnect the negative terminal to the battery until the very last step.

Sorry I don't have more knowledge of the wiring on the BMW.

Hope that helped.
Good Luck