POTA is a film developer. POTA stands for Photo Optics Technical Area. Developed by US military. It was designed for specialty films (like copy films) to yield near-normal contrast and extremely fine grain. I would only use it with such films, or when subjects contrasts are extreme. Yes, I have used it with Tech-Pan.
Phenidone is a developing agent. It's more efficient and stable, compared to Metol. The same level of development can be obtained in both less time and at a lower concentration.
I'm not a chemist, but I suspect that you cannot make Phenidone. Just buy it from a chemical supplier. I'm in Canada, so I order it from Jacques at Argentix.
Personally, I prefer to use D-23. It's very versatile. For fine grain, I use it straight. If I want a little more sharpness, I dilute it 1+1 to 1+3.