Not the same guy, but I AM into physical anthropology, and also happen to know that kidney stones are (to simplify) solidified excess calcium. So while milk is generally considered a good source of calcium, if it was the only source I don't think it would be an issue. If you have a calcium-rich diet as it is (other dairy, nuts, dark greens, soy beans) adding in breast milk probably won't help.

As for the adult nursing in early humans there isn't a lot of evidence to verify that other than modern tribal or "un-civilized" societies. A cursory search (on mobile, unfortunately) doesn't bring up any research espousing this theory, so, as compelling as it is, I can't vouch for its accuracy.

It's generally held that human breasts are a physical adaptation that mimics the prominent butts of monkeys and apes for an upright posture. Female simians have a thing where their rear engorges and the whole area gets bright red (it's a flush of blood to the sex organ similar to an erection in males, if I'm not mistaken) when they are sexually mature and in heat, which draws the attention of males and gives them a visual cue. You might notice if you have pets that quadruped females don't have pronounced breasts until they are nursing, but, since humans aren't quadrupeds, the male gaze doesn't have a straight shot to the baby maker at all times, so (the theory goes) females who's genes lined up to give them permanently large breasts were favored because of the visual cue that they are ready to procreate.

So, the attraction to full breasts, specifically in the case of this thread and sub at large, can be explained as the animal instinct to procreate being engaged by the visual cue of a fertile female. Typically women only produce milk when they are pregnant or nursing, so what better clue as to a woman's fertility than the absolute fact that she has clearly already succeeded in having a child.