Can someone check this so I don't fry someone's gps/transponder?
There was way too much to read, so I'm only responding to the first schematic.

You have roughly the right concept, but some details are wrong.

Let's check the resistor value.  The LED drops 2.1 V, and figure about 700 mV E-B drop in the transistor.  Starting from 5 V, that leaves 2.2 V across the resistor.  For 20 mA, the resistor would be (2.2 V)/(20 mA) = 110 &Omega;.  Your 120 &Omega; is therefore very close to the limit.  I'd use 150 &Omega; to provide some margin.  You'll barely notice the loss of brightness in the LED.

The outright wrong part is how the switch is incorporated.  When the collector of the transistor is floating, the transistor acts like a diode E-B.  The signal will still try to light the LED, but there won't be any gain from the transistor to help it.  Put the switch in-line with the base, not the collector.

A more fancy circuit would detect toggling activity, but what you show is one step better than nothing.