biconical shape is not better suited for some sling types than the others, but it's rather the aerodinamics and maximizing the range, by lessening the drag that they are best suited for. and ofcourse, if you can get bullet rotation, all the better (i think lobohunter describes how to get your glans go point-first, wich improves their penetration power)
any type of ammunition that's manufactured, and keeps same size and weight (is consistent) improves consistency with range and accuracy, simply because you get used to it's specifics, and don't need to consider different properties of your ammo for every shot.
we are sure that there were slinging "sharpshooters" (we have some written sources on this site), but maby it would be more appropriate to think of slingers as of artillery batteries, rather than marksmen, because they don't need to aim at certain enemy, try and immagine a hail of glans that could 30-50 slingers unsleash on the group of advancing enemies. (aim at the group, and you'll probably hit someone in it
)
try making a sling, and drie a few glans, so after you get the basics with a tennis ball, you can try and see what demage can you do with clay glans, and what kind of range, so compare it with the ranges on this site, and you could get a nice picture
p.s. it may be more appropriate if you say that neolithic glans FROM Tell Azmak measure an average ...., because i'm sure that someplace else, in the same period, the measures could be quite different.
hope it helps, and no, nicely structured questions are never tedious.