1. Someone is already working on it... I don't remember the name but it's been vaporware for a while
2. I'm not sure that would handle torque from different angles. Pulling up? Do you have any idea how much force goes through the pedal? On my ride today my max torque was 1800lb-in... that's 264lbs of force. I don't know if those kinds of sensors can handle that much load. I know my kitchen scale only goes up to a few pounds...
4. I don't think the Garmin would be able to handle to signals... I'm not sure if that means the ANT+ protocol can't handle it though. You may need to combine signals from both pedals and retransmit it. Also, power is torque x cadence so you also need something that can measure cadence.
5. 100??? What ANT head unit are you looking at that is $100?
Well as I said I spent few minutes (less then 5 I think) so I have no idea if someone is working on it.
2. Orthogonal is the keyword here, you measure vertical/horisontal push/pull, from that data you can extract true force vector. 264 pounds is nothing. Those can handle much higher loads.
4. Garmin will not be able to handle those directly. I was talking about having your own head unit. If one wants to use Garmin then either Garmin's firmware has to be updated, or extra ant unit with small controller has to be added to convert all signals to a single power value. It is easier I think to produce one's own head unit but yes communicating to Garmin would have great advantages since it does much more then just measure power and it looks to me that Garmin is well on it's way to become bike computer of choice for people with money.