Canon Fodder · 2:01pm Sep 3rd, 2017
I've been shot with a glancing blow from the Hasbro canon. I need to cancel my plans to extend Veneer and mark it Complete again. (Episode thoughts follow.)
I hope they can still do the tongue thing after changing.
The good news is that the recent episode actually confirmed some of my headcanon from Veneer.
The bad news is that the recent episode took at least two clever pieces of headcanon I was going to use for the story and made them actual canon (or close enough to it), so using them in the story now would simply be a rehash of things that are already set. 
The primary concept was going to be that the metamorphosis isn't solely the responsibility of the changeling. If you paid close attention to the episode, it wasn't simply that Pharynx needed to accept the change in order to change. In order to change, the other changelings needed to accept him, and recognize that he brought diversity and value to the hive even though he was different.
I like that message, and it was the place Veneer was ultimately headed. But I think the episode did a good enough job of illustrating the idea, so there won't be future updates.
However, some of the work I had done prior to the episode will be filtered into other stories I have to tell. So the interesting ideas I had won't go to waste. 
Side note: I don't want to ruin the Movie for myself, so I'm not reading the books yet, but I'm hoping the horn thing hasn't wrecked Twilight's Secret Journal canon. Even if it has, though, it'd be dumb for a horn to not regrow so it wouldn't really bother me. Older unicorns have longer horns, horns get filed down in the spa, and horns actually work by growing, so there's already ample evidence for horn growth. If Tempest can't regrow her horn in the Movie, there should be a separate logical reason.
I previously thought that unicorn's horns are constantly growing from the base and wearing off at the tip because of sole reason that it is the only sane explanation for their very regular screw-like shape. You've collected pretty large number of very convincing evidences. Cool! Although constantly growing screw-shaped horns should slowly rotate around their axis, which is not very compatible with curved horns from previous episode (but it may be just ancient legend).
From this theory there probably should be collection of "magical stem cells" somewhere near the base of horn inside unicorn's skulls, and if it is damaged horns could not regrow. Or full regrowth could just take a very long time.
Funny thing: I've just read about Rarity's horn in TSJ ~1 hour ago.
It'll probably grow back after a magical friendship wave of victory.
I missed this. My first thought was that it was mostly Pharynx's decision, as the change happens right after he declares that he loves the Hive. This ties in with the idea of giving love rather than just feeding off of it being the catalyst, as seen in To Where and Back Again.
I like your concept - you might still be justified in using it without fear of rehashing canon.
I might just be overlooking something though.
There were a number of oddities surrounding changeling lore introduced in the episode.
Aren't you all siblings? I mean, we saw Thorax hatch, and he wasn't exactly alone. Is there some other distinction here?
Apparently some changelings--and by "some" I mean approximately one--hatch (whether out of their egg or out of their pupa) with eyes of a different color than the rest of the hive members. Why? How?
Pharynx has antlers now. He's a "changeling stag," if you will, same as Thorax. Does this denote leadership? A higher caste? A particularly monumental transformation? Personally, I'd like to think it's the latter, but that seems pretty subjective. It also opens the door for Chrysalis to become a changeling doe, and I'm really not sure what to think about that.
You covered the others yourself, so I won't bother repeating them. It seems the more answers we get, the more questions arise. Do we want answers or don't we? From a fanfic writer's perspective, I see the benefits of both.
P.S. How many times has Thorax been referred to as "King"? It seems they prefer to call him "ruler" or "leader."
Maybe it just takes a while for a broken horn to grow back. I mean...
Also, I noticed I got something on your read later list!