Elpis flowers are entelechies. Entelechies turn potential into actuality ('telos' meaning 'end'). The ambient emotion that they experience is converted into a manifestation of Dynamis.

Everyone has emotions, regardless of aetheric composition. So rather than saying that Hermes changes the colour of the Elpis flower, it's more accurate to say that the Elpis flower responds to the ambient emotion that Hermes possesses and changes its colour by a dynamis-mediated effect.

The same is true for Meteion as well. As an entelechy, she experiences others' emotions as if they were her own (refer to the candied apples example). She is so influenced by the negative ambiance of Ultima Thule that she speaks on behalf of the dead ('We have suffered.') Our fight with her is not really about wielding Dynamis as a weapon the same way we would Aether. We simply alter the emotional state of Ultima Thule by introducing our hope, our beliefs, and our resilience. And slowly, Ultima Thule itself changes as a result through the power of Dynamis. This is more of a subconscious act than a conscious one.

The Aether limitation has to do with the fact that Dynamis itself is more subtle than Aether. As an analogy, in our own world, physics describes four fundamental forces - Gravity, Electromagnetism, Strong Nuclear and Weak Nuclear forces. Each of these are influenced by the scale that you're looking at. So while the Strong Nuclear Force may be much more powerful on a subatomic level, it might appear insignificant when you're talking about physical objects that we see in our day to day lives (which are dominated by Gravity and Electromagnetism). Dynamis effects themselves are less obvious in very aetherically dense regions, and very aetherically dense beings are less likely to affect or be affected by Dynamis (much like how a planet's movement is more likely to be influenced by Gravity than the Strong Nuclear Force). But as the aetheric density lessens in the outer expanses of space, Dynamis comes to dominate. Hermes is not able to influence Dynamis directly. However, by creating a being of suitable aetheric density, he can observe Dynamis-related effects manifest themselves, similar to how the Hannish scholars did by observing Elpis blooms (or similar to how humans use accelerators to create subatomic particles and observe Strong Nuclear effects).

On the subject of whether there's a connection between Faith and Dynamis, that's hard to say at this point. If you take everything that the Amaurotines put forward as truth, then Summoning is merely a derivative and inferior form of their own Creation magic. But they didn't really know about Dynamis in the first place. It's possible that, unconsciously, modern summoners have produced something with even more potential power than the Amaurotines previously envisioned, by linking Aether and Dynamis. Perhaps we'll learn more in Meracydia.

As to why some people are so reluctant to accept the discussion about Dynamis/Akasha, it really depends on the person. Dogma is always difficult to overcome, and if you introduce something as a 'rule' initially, some people can't move past the possibility that it could be wrong. Science perpetually requires you to have the flexibility to reject old theories. Personally, I really like the 'thought/feeling' dichotomy that it presents. It's also interesting seeing a clash of values between the traditional classical thinking that has dominated Western philosophy and Vedic mysticism/spiritualism.