1. The Echo vision with Cid remains unclear. Implicitly, the Warrior of Light was able to reach back through time to give Cid his trademark goggles - though if I recall correctly it was more along the lines of "speaking with Cid's soul" through the Echo back then, which is possible without some sort of time travel (such as Alexander) since it's essentially a dive into someone's memories. (Or: the Echo literally lets you observe with the memories of others, but they usually remain none the wiser unless they also have the Echo. Cid just happens to be astute enough to have figured it out, though it still doesn't explain the goggles.)
2a. Estinien explains that he struggled with Nidhogg for control after his defeat in the DRG 50 quest, but eventually won out.
2b. My reading of that scene is that the Echo was used to reach back into Nidhogg's memories and conjure an image of Haldrath - the real one - however briefly. It's been a long while but if I recall they both wore silver Drachen armor in the brief glimpses we get of them (one during the DRG 50 quest, one after defeating Nidhogg in the Aery).
As for the heresy... most likely he would be forgiven on account of his status and the fact he was obviously not in his right mind.
3. Probably just a joke referring to Rhoswen? Not sure.
4. Most likely just meant to clue people in to the fact that Mih Khetto's Amphitheatre is host to a good number of seasonal events (I took a break from leveling my Dark Iron Dwarf on WoW and jumped right over there to do the event - sure enough, that's where it was).
5. Not ringing any bells, so I'd need a sound bite to try to tell.
6. That's probably because, as I recall, Arcanists were intended to be a part of 1.x (1.0?) albeit using mechanical / clockwork magic staves instead of the grimoires they and their branching Jobs now wield. Urianger still having one in some early 2.0 scenes is probably just an oversight.