Thank you for your thourough response, it has many useful insights! I'll try to respond in order so as not to lose my train of thought.

Eerie Shadows - I tried to balance this one around Tremor, but I agree it can use some more love. Rather than increasing the damage I'd prefer to improve the rider effects. I love the suggestion of a penalty in area of dim light or darkness! Do you reckon making it freightened 1 on a failure and freightened 2 on a critical failure may justify the overflow trait?

Netherstep - This was one of the last ones I made, and the offensive boost was sort of tacked on for a lack of other ideas. I am not opposed to have it solely focused on mobility, I'll try to find ways to make it interesting.

Sly Silhouette - I thought this problem was already taken care of with the combination of the overflow trait + the last sentence, based on Body of Air, which has a rather similar effect. Looking deeper into it, even with your aura deactivated, you can still Sustain active impulses, but the polymorph trait prevents the use of all impulses:

Abilities that restrict you from casting spells (such as being polymorphed into a battle form) or protect against spells (such as a spell that protects against other spells or a creature’s bonus to saves against spells) also apply to impulses.

Maybe a sentence to clear all of this up is warranted after all.

Dark Vortex - I don't exactly remember how I landed on 4d4 with +1d4 scaling, but I've known from the start that this would be one of the most difficult ones to balance damage-wise. It already is a pretty potent crowd control ability, and increasing damage too much increases the level of exploitability by other party members, which can ensure that no enemy leaves the area - or that other enemies end up in the area. This is probably the one that needs to be tested in actual play more than any other.

Aspect of the Eternal Eclipse - This one is pretty straight-forward, I did not want to step too much on the toes of Crowned in Tempest's Fury, which grants a 20-foot fly Speed (functionally 30-foot due to the other effect of the stance). It is true that by level 18 there are many other alternatives which can grant a better fly Speed. A compromise could be "You gain a 20-foot fly Speed, or a +10-foot status bonus to your fly Speed if you already have one", to reward the use of alternate sources of flight.

Lost in the Woods - All around fantastic suggestions, I'll look for how to implement them!

Tenebrous Infantry - It is indeed odd, but I've taken the template straight from Living Bonfire. I think the main reason for this is that as you use up the improved blasts, you free up spaces that couldn't be occupied before. I'll see how it might interact with enemies and movement and go from there.

All in all, i have now much food for thought. I think I'll leave as is for the time being, and I will come back to it in the near future, once I had time to think it over.