Adoption of the cavalry shield in Hellenisitc heavy cavalry.

[MeinPanzer]

I can't recall the particular example I had in mind, - a silver dish - but while looking for it came across another example in Montvert's "Bactrian Armies vol 2", another silver bowl showing a lion hunted by an unarmoured rider with two-handed 'kontos' ( dated 4-5 C AD- which doesn't affect the point that the motif was not unique to Parthia/Persia).

I've never seen any sort of Graeco-Bactrian example of a horseman spearing a lion, which is a thoroughly oriental theme. The bowl you cite, aside from being entirely irrelevant to any discussion of the motif's nature in the centuries BC, was probably made under Persian influence, which was still strong in Bactria under the Sassanids. The fact remains that early examples of this motif are entirely Parthian.

In addition, that book also shows other examples of 'kontos' use e.g. fig 3(d) - another light armed 'kontos' wielder from a terracotta flask fragment, from Khorezm, 4-3 C BC; or 26(a), a 1 C BC example

The former is sometimes cited as an early example of a kontophoros, but if you look at the drawing, and other clearer examples from Tolstov's original publications, you will see that the left arm is gripping the reins, and does not run past the horse's neck (what looks like a continuation of it is actually the reins themselves). This is thus a lancer, but not a kontophoros.

I don't think your point is particularly valid here. In general a spearhead must be 'balanced', head to shaft, and thus the larger the head, the longer the shaft. One cannot judge from art, because depictions are not to scale, and of course shafts are seldom found intact in graves - being left out altogether, or snapped to fit it into the grave ( and for religious reasons weapons were 'slighted' - the real reason to make them unattractive to tomb-robbers no doubt).

As to the Saka you refer to, an artistic depiction is for reasons aforesaid untrustworthy as to size of head, and in reality a short-shafted, large headed spear cannot be wielded effectively.

We find massive spearheads in La Tene Celtic panoplies with only small or no spear butts, and also Lucanian spears or javelins with enormous heads and regular-length or short shafts are depicted on red-figure vases, including ones used by cavalrymen.

I'm not familiar with the burial you cite, but a weapon with large head and butt/counter-balance, over 10 feet long sounds like a 'xyston' ( 'kontoi' don't generally have, or need, butts, being held in two hands.)

Where is it said that kontoi don't have butts? What if the rider wanted it as a secondary weapon?

While on that subject, it is not to be supposed that 'palta/longche' dual purpose weapons went out of use with the introduction of the 'xyston' to Persian cavalry by first Darius and then Alexander ( for an illustration of a clearly 'xyston' armed Persian cavalryman c. 330-320 BC, see the akinakes sheath from the Chertomlyk tomb) or the introduction of the 'kontos' from the steppe nomads ( whenever that took place).

This is tangential, but the Chertomlyk scabbard is a very poor iconographic source. It shows a highly classicized amazonomachy, and that figure on horseback is almost certainly an amazon. Whatever details were drawn from real life (and the xyston and saddle of the dragged amazon on the right certainly look true to life), this cannot be taken as a representation of a xystophoros Persian.