Ivalice Raids. I still don't get it

[JeanneOrnitier]

Why are there Lucavi waiting around in the ruins of Rabanastre and the lower floors of Ridorana? Like, I know auracite from Ultima causes people to turn into Lucavi, but why are they there, waiting for us to encounter them, and apparently doing nothing else worthwhile?

I assume either:
a) people who used the Auracite back in the War of the Lions and have been there all along
b) people who found the Auracite recently and were whisked away by Ultima like what she did to Alma

The first option works for Lesalia though not for Ridorana since the Dalmascans used it as a lighthouse.

Who was the guy who turned into the final boss of the Rabanastre raid? He ends up being named "Argath Thadalfus" when he auracited on us, but who was he before that? Why was he just waiting at the bottom of Lesalia, an ancient mythical city thought lost to time? How did he even get there? (The lorebook also asks, but does not answer, these questions.)

He's the ghost of https://finalfantasy.fandom.com/wiki/Argath_Thadalfus one of Ramza's minor enemies from the War of the Lions. In Tactics, after Ramza kills him he strikes a deal with the Lucavi and becomes undead to get revenge, which is probably why he asks for Ramza to forgive him in XIV once he comes to his senses.

When Ba'Gamnan shot his subordinate at the end of the Rabanastre raid, and that subordinate just hopped back up apparently unharmed, why did just about every poster here, including those more familiar with Ivalice, speculate that it's unusual and clearly supernatural? Given that in the very next tier, the explanation given was "well, Bangaa are just really hardy, don't worry about it". So was it a plot point that was just dropped?

That was just the players reading too into things.

Speaking of dropped plot points, was there actually anything significant to Ramza Lexentale's change of attitude at the end of the Ridorana tier? Because even the NPCs commented that Ramza is acting completely different, but when the Orbonne tier was released, it turned out that Alma (who was acting pretty normally) was the one under the influence of the auracite, and Ramza explained his change in behaviour as "I was just really stressed", and everyone just dropped the issue. Was it another altered plot point?

This one does seem to be a change yeah.

If the spell Ultima was named after Ultima the High Seraph, does that make Lahabrea's comment that the Ultima spell in the Praetorium "a sliver of Zodiark's power" still accurate? I mean, as an Unsundered Ascian, presumably Lahabrea should know the truth. Is there a link between Ultima the High Seraph and Zodiark?

The spell being named after the being is just speculation on the characters' part. That being said, wasn't it the Heart of Sabik that was a sliver of Zodiark's power, not Ultima? I'd need to double check the exact line.