Aren't shadowlng just giant brains held in a round black bodies? The shadowlings don't need a spine because their arms are practically attached to their bodies. It is like not using a spine to use your eyes.
Their lack of a central nervous cord is actually very important if we're looking at evolutionary similarities. Spines are only possessed by animals in the
phylum Chordata. Phylums are very high-level divisions; organisms in other phylums often have radically different physiology. This is especially important because we're talking about brain chemistry; organisms without spinal cords must have different ways of transmitting brain signals to their bodies. Even if their brains have the same receptors, it could have a different effect, or they could use a subtly different neurotransmitter.
However, since the skills do not magically produce the necessary chemicals, only entice the body to produce them, it is possible that Stimulate simply releases a different chemical for shadowlings that has the same effect, if necessary.
(In fact, this raises an interesting question: do shadowlings produce adrenaline at all? Since they are so psy-affine, it would make more sense if their bodies released some kind of psy-boosting chemical instead.)
You are putting way too much focus on the smoke part of their bodies. The smoke on a shadowling is like hair on your head. Saying a shadowling is "made up largely of that intangible smoke stuff" is like saying a bear is made up largely of fur.
Hm, I suppose that is true. However, how can you be certain their heads are the same as human flesh? The fact that they refer to mammals as "fleshlings" implies that they do not possess it. In fact, judging from how they call themselves "shadowlings" and are naturally affine to the shadow element, it is not completely unprecedented to assume that they are, in fact, made up wholly of some kind of shadow matter.
Edit: Just to fix the bbcode (there was a "=" instead of a "]"). Ert.