Most of them are just special quirks I added so the characters would feel more unique. Since they're pre-gens for a one-shot, I felt more flexible to do odd stuff and not worry about balance. These are all high-level characters, so I just imagine they got a bunch of unique boons over the course of their careers, like any good PC would. Amielle is 'immune to curses,' for instance, which is something I just think is fun for Tieflings to have, especially in this adventure.
I wanted to add Engineering as a skill for 5e, but realized while working on the Player's Guide that the system didn't *really* need it to be a skill. It probably ought to be 'engineering tools,' which is what Technologists get, though she's also the archetypal Gunsmith, so she ought to have gunsmithing tool proficiency too.
I like the flavor of setting someone on fire more than 'here, take some damage' that Hellish Rebuke and every other fire spell uses. Like I said, I used BoFF to tinker a bit.
The Rites of Rulership are intended to let you fight huge monsters, so they grant actual bonuses instead of just advantage. The rules for the rites in 4e and PF already work pretty much like that.
Tomas is a weird sort of hybrid monk-paladin. Again, Tinkering.
Lee's power was supposed to work just like firebolt; I just forgot it scales at 5th level.
Lorcan's dagger probably should just be +7 to-hit, since it's not magical. The bow benefits from Archery fighting style, which grants +2 to attack rolls.
I think the Ascent rules were a spot I didn't notice I needed to do a conversion. Yes, it should probably be advantage and disadvantage.