MPPT using preassembled 'adjustable' buck/boost IC
No.

The switching frequency of the LM2596 is 150kHz, but the sampling frequency of the Arduino is only 10kHz (on a good day).  You won't be able to adjust the PWM quickly enough to fine-tune the LM2596 output voltage.  The LM2596 uses the 1.23V feedback voltage to change what it's doing.  But the feedback voltage will only change as fast as the Arduino can change the PWM feeding it (and that the RC smoother will allow).

It will be like pushing a shopping cart backwards, blindfolded.  By the time you feel the directional change in the cart, it will have steered off course by quite a bit.  Then you have to compensate wildly to bring it back on course.  

The same would probably happen here: the LM2596 would begin adjusting the output voltage to make the feedback voltage come back towards 1.23V.  But the Arduino doesn't respond in time.  The Arduino has to do an ADC conversion, do some calculations, change the PWM, which eventually changes the RC output voltage, which is the feedback voltage.  So the LM2596 changes some more... and even more.  Eventually the feedback voltage does change, but by this time the output voltage has changed significantly.

Another way to look at is that the LM2596 is like another "processor", trying to optimize its own function, output voltage instead of MPPT.  It's running much faster than the Arduino, so there's no way the Arduino can trick the LM2596 into doing what it really wants, MPPT.