Don’t know about those sources of yours, but trisodium phosphate is a solid and cannot be used as a paint thinner—only solvents can act in that role. Being a salt of sodium, though, its more common uses are in saponification (making soaps/detergents by reacting with greases/oils), as a cleaning agent (especially degreaser) and de-glossing of oil-based paints in order to make new coats of paint adhere better. Its food use is (reportedly) as an antioxidant.
Propylene glycol has very low toxicity compared to ethylene glycol, the most common automotive antifreeze; as far as its use as a RV/marine antifreeze, the low toxicity and low potential for environmental damage is the big safety point most touted.
Incidentally, ethanol (booze) is also used as a paint solvent and thinner, and of course as automotive fuel. Does that mean we ought to go Sharia on booze because of that fact?