Sounds like you are having fun with your new wheels!
Good catch on tracing the tx interference to your mic extension cables, and thanks for your update.
I just wanted to comment and clarify on a few things. You are correct in the fact that the entire electrical system in the car does not "go to sleep" when you shut off the power button on the dash. There are several electronic control modules (ecm) that will stay energized for a period of time. However, when you shut off the power button on the dash, it does shut down the high voltage system in the car, and it isolates the high voltage battery from the inverter/converter assembly by de-energizing the system main relays (high voltage contactors). I thought maybe you were getting some interference from this system, and by turning off the power button, this system would be eliminated. If you haven't experienced noises coming from the car when it is shut off, you will somewhere along the line. I've been in the garage hours after the car has been powered down, and heard it make sounds. While the conspiracy theorists among us would like you to believe the car is haunted, it is normal operation. Also, you'll notice when you open the drivers door and go to get in, you'll hear an electric motor precharging the hydraulics in the braking system. I agree with you in that this isn't the 66 mustang anymore! Despite this, there really isn't anything going on in the 12V portion of the Prius that isn't in most cars today. What I mean by that is that I'm still not convinced that the tx noise is being caused by the electrical system of the Prius. I know you've already moved the radio, but out of curiosity, did you try a different antenna, maybe a mag mount in middle of the roof and see if the tx noise went away? You could plug in your mic extension cable and retry. I'm just wondering if perhaps something with the mounting location, or perhaps an inadequate ground plane from the antenna mount is causing the grief? This experiment could help eliminate that.
As for the back up beeper... That was a pain in the butt! I'm not sure why they design the alarm to work that way. It reminds me of the back up alarms on our farm trucks, except its beeping inside the car instead of outside! I have access to a scan tool, and I made quick work of that. Now, it only beeps one time when I put our car in reverse. I know from the prius chat forum, some dealers charge to remove this option, and others will do it for free... A little sweet talk may go a long ways.
Also, on the ev, eco, power button: While it looks like there are only three modes of operation, there are four. When neither EV, eco or power is illuminated, the car is in "normal" mode. What does this impact? First, and most noticeably, it impacts the relationship to throttle opening on the engine to the movement of the accelerator pedal. While the car won't make any more power in the "Power" mode, it is more responsive as you step into the accelerator pedal. Second, it will impact how aggressive the car cools and heats the passenger cabin. Eco will be the most sluggish to drive, and slowest to respond to climate changes via a/c or heat.
Hope this info helps, and keep up the good work. Happy motoring!
Wane - KB7FSC