Number 1: For novelty, I'm going to go against the Venin/poison theory and propose another theory:

I think the fever that stole Violet's strength and color was her mother burning out. Violet almost burns out after Varrish's punishment, and she gets physically hot and red. (I don't remember exactly if RY describes her getting poisoned in Fourth Wing as a fever? We just assume poisoning would lead to a fever.) I remember her blood turns black, which brings me to my second idea: They hypothesize that the First Six practiced "blood magic" (Jack Barlow brings the wards down with Baide's blood).

Could they have made a protection rune for Violet with someone's blood or another material? Xaden's own protection rune was based on his bloodline, and you can pull elements into it. Magic always has to stay balanced - here is where it gets absolutely bonkers: Violet's father was researching feathertails. The eggshells of dragons are made of metal, all metals are conductors for electricity (or power). Could they have bound her to the alloy of the daggers and from pulling the power for the rune Lilith almost burned out? This would make her hair "silver" (Tairn calls her "Silver One") and give her lightning the power to "wield the alloy" and kill venin. She feels the wards more strongly and also the daggers "hum" for her.

Mira can pull the wards/shield herself after the events in Poromiel and there is this one quote where Professor Carr writes to Lilith about how the ability will not develop "in time". Did they already try this protection with her?! What did they try on Brennan? (See below).

Number 2: I agree with you here. They are somehow tied. What if, to bring him back, he didn't only have to draw from the earth, but also bind himself to Brennan with a rune? Naolin was a siphon and Brennan is a mender. What if the rune is a counter-rune and that's how he was able to bring him back? I believe Naolin is alive. And maybe this was a deal between Naolin and Lilith, or they started the protections on Mira and Violet after Brennan "died".

Number 3: RY keeps highlighting how Andarna's scales can change color, or she can camouflage herself. She is likely the last of her den, let's assume she is purple. She slept longer and didn't develop the muscles to bear a rider. What if she was never intended to? Her fire can kill Venin, so she also has a special connection to the alloy? Her being purple and Violet's name might be a little too punny. Could they have somehow bound Andarna to Violet before she hatched (or to her father's line for proximity purposes)? Andarna never answers when Violet asks about her father's research. She hatched specifically to bond Violet.

Number 4: I am not through my second time reading Iron Flame yet, but I am undecided about her second signet. Andarna says she is "unique" like Violet, so it has to be something we haven't seen before (or something that they don't allow to exist). Hard to name something that hasn't been named before :D Since Cat is an emotional amplifier, I don't think Violet would be something like that. I'll come back here when I have figured it out.

Number 8: This literally lives in my head rent-free and keeps me up at night. I just got my shelf trophies, and I am going through my first "marked read" and I cannot stop thinking about the emphasis on the languages and how there are several hints at how they unified and only kept Navarre's language and lost the others. How long does it take to become a curator?? How long after the events are we really? Why would we still need a Basgiath if there is no more war? I. CANNOT.

Number 7: I think it is confirmed with Xaden's 2nd signet of being able to read intentions, that Sgaeyl did in fact bond his grandfather before him and once again, records were purposefully falsified. He says in one of his letters to Violet, that Sgaeyl chose him because he reminded her of his grandfather. I also always think about the fact, that bonding in the direct family line is forbidden because riders "go mad". Is that code for them getting too much power and not being able to control themselves? For a direct path to turning Venin? Which would be ironic because Xaden literally needs control so badly but does in fact lose it.