All this week the PCBA with detector circuit was attached to my microwave oven. And guess what... at some moment I noticed, that sometimes LED is emmiting light!!! After some observations I could say, that energy leakage beyond the glass is dependant on food type, quantity, shape and rotating plate angular position. I managed to capture some video with the help of le croissant:

in the dark LED is visible much better:

At this point, IMHO, LED can be desoldered from PCBAs.

Moreover, I managed to assemble the PCBA part with energy harvesting IC,  finally!

I made three prototypes with three different turn ratios of transformers: 1:20, 1:50 and 1:100. Unfortunately I forgot to order some of ceramic capacitors, therefore I used slightly different values or combined some other values in series / in parallel.

Don't know, whether due to that deviations from nominal (because there is strict oscillation frequency range for the circuit required), whether due to some other bug, but none of these PCBAs worked as it should :\ - using voltmeter, there is no increase of voltage on output of IC.

also I noticed the bug in silkscreen - GND and VOUT2 text near header is swapped - need to fix this in next version.

So the next step will be debugging - I'll try to change component values to their designed (already found some 0805 capacitor mix, which I'll manage to solder in). If this doesn't help, I believe that power supply and oscilloscope will be enough to detect and solve the problems.