First I have to say that I have mixed thought on Praxis and Theory's quest, their quest were obviously not perfect but it wasn't all bad either, while it is good for Blades like Praxis, Theory and Vess where you can get theme by completing their quest and without have to go with the RNG, but here's the thing...
My first bother some on these kind of quests is that once you know "Blades will return to core crystals if driver dies" and "Blade will lose memories after being resonated. Once you saw Praxis, Theory and Vess, the ending of the quest is already predictable and their drivers will have instant "death flag". Which is why I aware that Vandham will die even before I finish chapter 3, and so is Vess' driver and both Praxis and Theory's driver (And yes, even how Theory's driver dies it was a good result to me but wasn't that much surprising to me either because I already knew Blades can also kill their own driver by will from another quest). Not to mention you have to wait for a long time to resonate those Blades, and took even more time to complete Praxis affinity chart and get pass to chapter 6 in order to get Theory... That's why Theory ended up being the last rare Blade for me to get (Even after I got KOS MOS LOL)
Now off to my Praxis and Theory's #1 quest review and a bit of rant:
There is a line of Blade Quests revolving around a pair of sister Blades, Praxis and Theory, who are stealing Core Crystals for a group of criminals, often killing people to do it. You spend a while, with aid from Mor Ardain's military, preparing to set them up for an ambush in two separate locations. Praxis and Theory arrive in one location along with Theory's Driver, but Praxis' Driver arrives in the other location. Praxis' Driver gets killed by the Mor Ardain military off-screen which results in Praxis returning to her core crystal. When a Blade returns to their core crystal, they lose all memory of their previous life. Theory and her Driver flee the scene, while Rex's gang takes Praxis' core crystal. You can resonate with it after a bit of time.
It was expect but interesting as to how Praxis and Theory are "Blade obtain quest", at first I was hoping that Praxis and Theory might have some understandable reason to steal core crystals and as you mentioned they even have to kill people to do it. The bad part of this quest is not that Praxis and Theory were antagonist, no, I'm okay with that, what is not good is the core crystal hunter's motives, which they never had a good motives to do so thus in my own opinion this somewhat ruins Praxis and Theory's character even as a villain Blade. I mean, sure Blades will have their personality affected by their driver so it makes sense that they will listen the core crystal hunters but it's odd how they will just do almost anything that core crystal hunter tells them to do and not even having any skepticism.
What also kind of odd was that Praxis and Theory aware they were being chase down by Mor Ardain soldiers (And along with your Blades that sent via Merc mission). Why Praxis wasn't with her driver? They should either all go and battle with Rex or Praxis goes to the other location with her driver. Wat I will change is that Praxis and her driver goes battle with Rex and lost, and after we go to the location where soldiers are fighting, Theory and her driver retreats, now that's slightly better in my opinion. Because no unexperince driver will go battle without their Blade around.
That's where it leads to quest #2 which in my opinion is the best overall but still some flaws.
After resonating with Praxis, you can start a Blade Quest where Theory spies on your group and is trying get Praxis to remember her past, while Rex's gang keeps that info from her. After it's made clear who is sending Praxis the letters, Rex decides to tell Praxis about her life of crime and bloodshed. Praxis can't remember any of it due to the way Blades lose their memories when returning to their core crystal. However, she wants to connect with Theory again as she feels an innate sense of familiarity with her. You then confront Theory and her Driver and eventually, Theory kills her own Driver so she can be with Praxis again.
Like I mentioned, not just after resonating with Praxis... you have fill up her affinity chart to the point where she can start quest and the quest only available after chapter 6. Which is a bit too long of a gap imo. Just a minor complaint for me right there. Also snce you can don't resonate Praxis, and while yeah you can't get Theory in this way which ok... but it will be pretty interesting if there's like a second scenario where what happens if you don't resonate Praxis but still can get Theory... I know that's a bit impossible but just my thoughts. Which you might have to go to Theory and the core crystal hunter's hideout or something and have Praxis' core crystal on hand and try to defeat Theory this way.
This is probably Theory personality, how she just "spies" Praxis was pretty confusing, as a core crystal hunter she could just go and get them (But we know that's actually her last resort) but the infos she gives was actually hopes to let Praxis remember... then again what bothers is how Theory still doesn't understand how heinous her driver was since he didn't want to save Praxis he also wants to kill all of the crew and get their core crystals (Again... for what ever reason!) and it took a battle for her to realize that...
It's very very interesting to see how Praxis has her personality changed to some extent and how she reacts when Rex told her she was with the core crystal hunters and was once a criminal. And not to mention she has innate familiarity on Theory, which while it was odd for me at first but we have also seen with Vess a bit too anyway. She seems to be evil and likes to collect spears but now she's chatty and messy....And last but not least, yes, we never understand this but it seems that Blades can kill their own driver from their own will... this is both dark and confusing since you have some Blades being resonated by monsters, they could try to kill the monster if they can.
Man if possible I ... actually have a alternate idea for this, I hope Theory and her driver end up giving up their tole being core crystal hunter and join merc group, and then we have a quest where we search Theory after they go on a dangerous merc mission and then we find them nearly got beaten and Theory's driver dies and Praxis keeps her promise with Theory, I think that will work better, again just my own thoughts.
The third quest was probably the worse imo... not only that it took a battle and a conversation for those folks in Uraya to understand Praxis and Theory are not bad anymore. And then how their quest plays out is just... so bothersome to do (Praxis doing eyes of Shining justice was cool though, that really sums up that Praxis is a perfect cannon Blade for Zeke imo )
And oh man, I actually hope Perceval can be a Blade obtain quest Blade now... It will be nice if we have a quest where we stop an assassin Blade and then we do Perceval's regular quest. This will be better imo.
And so is Kasandra, it will be even more interesting if she has Blade obtain quest instead we spend times of salvaging in certain places to get her... why don't we have a quest where we help her and her Nopon driver? And then the quest ends up she and her driver end up getting eating by the mosnter lol, this will be both hilarious and dark.
So yeah... sorry with this nonsense rambling, but when it comes to villain amnesia troupe, Praxis and Theory's quest while it's a unique in the game, it wasn't really that well fleshed out in my opinion. Like I said, them and their antagonist driver weren't good, and the quest were pretty bothersome to finish too. But hey, don't get me wrong, Praxis and Theory are still good Blades it's just that I think they deserve a more better quest.