We are on the same page! Some clarifications:
I assume Vcap is the capacitor on the Solar Panels side?
Power = Vcap * (Vfet/Rdson) / 2, correct for the panel side.
Inductor Current is a ramp must Divide by 2 for the average current:-)
As the panel is loaded Vcap will change (a lot), therefore power is dependent on it.
As you state Rdson cycle by cycle is the same (dependent on temperature)

The energy in the inductor is the same for the charging and discharging equations:
Power = Vbat * (Vfet/Rdson) / 2 (if discharging current is a ramp)
Since Vbat is constant only need to max Vfet to Max power!
If discharging is not ramp not sure if valid comparison? Need to think about this.
Time PWM cycle to keep some current in the inductor at the end of the cycle to get ramp.
(As long as the load side conditions remain the same, expect OK to let current go to zero.)
Then comparisons of peak currents are representative of more energy = more power.
Power transfer with inductor current going to zero requires larger Peaks currents to get the same average. 4 = 8/2, if current goes from 2 to 6 = 4 same average current.
Peak currents are the root of most problems in a switcher.

Timing the length of inductor output is more difficult than measuring Vfet once.

Do you have an analog solution?
No, I could...
Couple years ago saw a very complex circuit doing MPPT. Much more Costly.
May have been in a patent. (where I go to learn how things work:-)