When calcium is present with phosphate at or near mash pH a tiny proportion of that phosphate will be in the form or the tribasic ion (PO4)-3. The various phosphates of calcium are very, very insoluble. Thus hydroxyl apatite precipitates removing some (PO4)-3 from the mix and upsetting the equilibrium between H(PO4)-2 and (PO5)-3. To restore the equilibrium some H(PO4)-2 --> H+ (PO4)-3. The relative quantities of the two species change and the pH goes down both of which lead to restoration of equilibrium.
There is lots of phosphate present in malt, most as the monobasic ion H2(PO4)- it reacts with calcium in several ways but the dominant reaction is
10Ca++ + 6H2(PO4)- + 2H2O --> Ca10(PO4)6(OH)2 + 14H+
Thus 13 protons are released for every 10 calcium ions that react. Kolbach's observation, mentioned in #4, was that calcium ion concentration [Ca++] would produce [Ca++]/3.5 H+ ions at knockout making it plain that most of the calcium present does not participate in the reaction as given. But still some H+ is produced and the pH is lowered. Suppose you have mashed a kg of typical base malt (Crisp MO) with 2L of water with 2 mEq/L (100 ppm) permanent hardness. You might expect mash pH of 5.72. Adding 5 mEq/L (100 mg/L) Ca++ is 10 mEq for the 2 L and, according to Kolbach, would release 10/3.5 = 2.86 mEq protons. Given that this malt has a buffering capacity of -46 mEq/kg•pH at pH 5.72 we might expect a pH shift of 10/3.5/-46 = -0.062 pH at knockout (in the mash it will be less than this) and so it is clear that calcium addition is not the way to control mash pH but it is also clear that if you have a lot of calcium present you need to consider its pH reducing effect in making pH estimates. It is interesting to note that the paper in which Kolbach offered his observations is really an appeal to his fellow brewers to get together and pressure the authorities to allow the use of external acids for pH control as is done by brewers everywhere in the world except Germany.
Finally, a salt, such as CaSO4, has pKs only in the sense that its acid does. In this case the acid is clearly sulfuric acid which has pK's of -10 and 1.99. That second pK lends SO4-- some alkalinity at mash pH but it is so small that it can almost always be ignored. Water with 500 mg/L SO4-- would have alkalinity of 2.6 ppm as CaCO3 with 1 ppm attributable to the sulfate ion.