I was all set to process my 2kg of borax as you describe (except I was planning to return the supernatant to the boiler each time for the next 500g so's not to lose the 47g/l) when I went to Google and found the MSDS for both brands that I have here. They are both described as tetrahydrate 99/100% depending on the brand.
I put away the kettle and borax.
My final move would've been to cool the last supernatant in the photo fridge to get almost all the borax back.
Selling tetrahydrate makes sense commercially - why give the customer the more concentrated pentahydrate if one can sell some water with it?
The P-C-Borax does indeed work well. I especially liked it with Copex rapid as a stand developer. I just don't know how long it lasts, made up. I used it as a one-shot. Borax is much more soluble in glycerine and ethylene glycol - maybe it's possible to make a concentrate?
Murray