Best way to adjust/set SMPS comp / compensation parts

The first point is correct. As a general rule of thumb, the loop crossover frequency should be a fraction of the converter switching frequency. I've heard both one-fifth and one-sixth cited as the maximum.

Crossing over at -1 is not a firm requirement for stability. If the closed-loop response at the crossover frequency has sufficient phase margin (typically 45 degrees or more) and gain margin (-10 dB or more when the phase is at 0 degrees), the converter is stable.

The reason -1 slope is cited as the target for gain crossover is that, in general, the phase is small and isn't changing rapidly during that part of the curve. When the gain slope is -2, the phase shift is greater and changes rapidly, making it difficult to ensure phase margin if the crossover frequency shifts (due to component tolerances, etc.) That doesn't make it impossible to choose a compensation network, but it does make it more sensitive to changes.

Strange. This is a single supply module, yet it has two switchers on board.

Anyway. Yes, connect a voltage between 3.5V and 28V to the input and measure the output voltage, which you indeed should be able to adjust with the multiturn potmeter. Check the switchers' datasheets for the maximum output current. The right IC seems to be an LM25005, that can deliver 2.5A. May be a bit high for the coil though. The left one I can't read.