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?