The readings are to be expected, including the polarity reversal. The resistive load will probably increase your 12 volt output and the differential should be very close to 7 volts. Most of the 5v rails read in the neighborhood of 5.05 to 5.15 volts. You can sometimes push the 12v rail up to 12.2 or so.

However, I can't provide a reason why your heli won't run with the 7v input. You need to be mindful of motor heating -- without a battery to run down, don't try to run too long without a cooling off period. Secondly, silver satin phone line is pretty heavy with the outer insulation sheath -- you can drop some weight by peeling the outer insulation away and using just 2 of the inner wires. Lastly, the wire gauge in phone extension line is very light and may not carry the current needed by your motor, especially over a long distance.