Questions regarding a micro-wattmeter
D1 and D2 are biassed by the same current. That means that if the diodes are at the same temperature, and D1 sees no RF, their voltage drops will be nominally the same, and so their difference voltage zero.

For small incident RF power, the voltage difference between the two diodes will change as the power. For large incident power, the rate of change will switch to voltage, as the diode starts to peak rectify. 

Whether you deem this to be 'overloading' the diode or not depends on whether you're trying to read the meter simply as linear power, or whether you understand that the input power to output voltage breaks away from linear power at some point, and have calibrated your curves accordingly. The 'voltage detection' region of operation of a detector is a perfectly valid and repeatable place to operate, and you would be throwing away a large part of your operating range by ignoring it. 

I would deem the amplifier to be overloaded once the second preamplifier approaches its P1dB output power and starts limiting the signal, causing a level reading error.

Incidentally, there's nothing 'magic' about power detection or voltage detection for a diode, it's just the way the physics works out. I did the exercise a while ago of modelling a diode detector as a pure Shockley equation, and integrating the voltage and current over one cycle. Sure enough, at low levels it's square law, transitioning to an asymptotic linear law, with a smooth transition region between.