Well, under rare circumstances, some humans have been able to perceive Hellions somehow, especially when their lives were threathened, is possible that she was able to seen that only for that short moment of time because what happened, but when she meets Sorey, it doesn't seem like she ever saw something similar before, so... even if you try to explain it, it doesn't make enough sense, it isn't something anyone would forget.
Well, about her oath... that's what the party of main characters theorized, but we never heard her oath directly from her or anyone close to her, so we actually don't know if it was really was her oath was about, also, even if the oath was to not kill, maybe it can bypassed by certain things (like not being the one killing directly)...
Well, we didn't know of any other dragons, but it isn't mentioned anywhere that there are no other dragons, only that we didn't see any around, and people who said to see one... was only right about the one at Rayfalke (Eizen), about why he was buried... no idea, a comfy place to sleep for him? maybe it was an Earth Seraph?
Well, he's not too weak, and Symonne wouldn't be particularly powerful without her illusionisms powers, but I agree that Symonne is able to kick his butt with ease, isn't that what got Lunarre to work under her at the game?
Zaveid's introduction in the game and unique design, together with his stranges goals and gun, shows that's important, only not in which way or how much... and I feel sorry for Edna getting barely any attention at all in the whole opening, though her being my favorite may have something to do... girls with a parasol are just too strong against me, ask Mio Nishizono too
I hope this... helps? Theorize or try to untangle this mess, because I was highly confused too.