These compounds are salts of phosphoric acid, only differing in the number of H+ ions replaced with K+ ions. When in dilution the different phosphate anions (H2PO4-, HPO4-2, PO4-3) transmutes between each other in function of pH, so they are roughly equivalent.
Potassium absorbability is as good as for any other source of K+ ions, like KCl. So no problem here.
Acording to this source the bioavailability of phosphates is high.
Inorganic phosphate is readily absorbable.
Sadly things aren’t as simple. An ionic compound dissociates only partially, the degree of it depends on its chemical nature and the composition of the disolution. Strong acids, bases, and their salts dissociate more that weaker ones. Acids dissociate more in an alkaly disolution than in an acid one.
The phosphoric acid is weak, so potassium phosphates dissociate weakly when in disolution. This is not a problem because as the ions are absorbed, more compound is dissociated to mantain equilibrium.
And that idea is indeed very clever. In theory a disolution of potassium bicarbonate and citric acid (needed to transform the bicarbonate ion into H2O and CO2) should be chemicaly equivalent to a disolution of potassium citrate. By the same logic, disolutions of potassium citrate and sodium cloride should be equivalent to disolutions of sodium bicarbonate, citric acid and potassium cloride. This may open avenues to reduce cost, albeit not by a significant margin. Other acids can be used in place of citric acid, like acetyc acid (present in vinager), lactic acid, or even gluconic acid to mimic the taste of potassium gluconate. Cost wise the best option is to use an ingredient with an acidic nature that is already in the recipe.
If only we could find an easy and simple way to synthesize gluconic acid from glucose…