Questions on load modulation, Doherty PA
Consider that in general a load is measured at some point in a network, and can be defined as load Z = V/I at that point in the network. 

In an efficient amplifier, we want the load seen by the power devices to be such that the devices are as close to saturation as possible on a moment by moment basis, which if you think about it is saying that the drain load impedance should be inversely proportional to power output.

In a classical power amp the drain impedance is fixed at a level that is low enough to support whatever the maximum power the thing is capable of, which is fine for a constant power mode, but horrible for anything with a significant AM component (Like say just about any efficient modulation scheme) as the input power falls as only the square root of the output power backoff (This is an inherent property of a fixed drain impedance/fixed power rail design).
Doherty can be thought of as using the peaking amplifier to lower the drain impedance of the carrier amplifier during modulation peaks this allowing the main amp to run a higher native drain impedance and be more efficient over the vast majority of the time when peak output is not required. 

Due to the 1/4 wave phase shift, they are inherently narrow band devices, but IIRC the first ones were indeed built for ~100kHz long wave service by Continental transmitters using valves as the active devices. 
A 90 degree phase shift does not need to be a transmission line, you can do it using a lumped element hybrid made from a coupled inductor and two capacitors, still narrow band, but workable down well into the audio band if you really wanted to. 
There are easier ways down that close to DC with modern parts, PWM with a GAN switch will make very efficient power up to at least the medium wave region without the narrow bandwidth of the Doherty designs.