One can experiment easily with different Katars and I am pretty sure the conclusion would be that a Katar with longer and stronger arms like the one in Jen's posting no. 7 is much more stable in the hand than one with shorter side arms like the one in Jen's posting no. 20.
Longer arms would ensure better alignment of the blade with the forearm (notice how the longer arms slightly converge opposite to the blade in order to give a firmer prop against the bearer's arm - the same reason why some Katars have the side arms slightly bent inwards, not as a result of a blow, but to ensure better contact with the arm) and through their weight will also serve as a counterbalance to the blade improving the handling of the Katar. The wide four transverse bars will also contribute to the stability of the grip and prevent the rotation in the hand.
The same thing cannot be said for the second Katar that would be rather difficult to use as the very short and rather widely spread side arms would offer no alignment and balance to the blade. At the same time the two transverse bars would ensure a rather narrow grip prone to rotate in the hand.
So, the first Katar would definitely be a functional weapon, while the second one would be more like a dress Katar. And here I contradict my own statement above (second part of it) when I said that "there are some Katars more suitable for combat than others, however, this doesn't mean they were deliberately designed for combat."
As with regards with the illustrations, they are extremely important for general assessment of the presence and use of the Katar on the battlefield, but I believe they are of less value for making an accurate assessment of the proportions of the weapons used, as the artist's focus was certainly not on illustrating the precise proportions of the sidearms.