Lemme add a few nickels.
Valzira may be too full of rage and hate to care about deception. Yes, it also clouds her judgement, but I suspect she's beyond lies now. Granted, whatever she says is warped - but not by an attempt to deceive you. IIRC, even during the stupidest gameover of granting her freedom after conquest she tells you she will fuck you up before you release her.
Malice is an interesting - and VERY cute cookie. INSERTING MALICE PIC NOW!
Ahem. So, I think that in terms of thinking she is somewhere between eldritch and fae - classical fae that is: unpredictable and capricious, less malicious and more on their own wave (which still makes them incredibly dangerous). Eldritch part comes in form of her plans and motives potentially being so far out there there is practically no way of predicting or, perhaps, even understanding them. She is insanely powerful capricious otherworldly entity who break reality just by being around. She may even have no plans or that her plans are so eldritch there is no point explaining, or that she's just messing with her dad by not telling.
No real way to tell anything for sure, really.
Regarding cat sisters - no, their fights are no play. Its very clear once you see their Sunken Temple scene. There is conflict between them, it's just buried very deep.
And final nickel would be Issa.
No, he's probably not saying "we" in regal sense - otherwise, why he always self-correct when he does that? Maybe there is some very specific reason of him not wanting to show his higher status, but there is no hints and no threads to pull. At the moment, this is empty supposition.
The other reason to speak that way, the possession, does not have direct proofs either. His body smells of heat - granted, its a hint at demonic, the only faction associated with heat - and petrichor - or, speaking plainly, that specific smell of dirt after the rain. The first smell hints at demons, the other... No idea, really. His blood emits no smell, but seems acrid - what that suppose to hint at, I also have no idea. Blurring over his corpse may hint at something incorporeal leaving it - or just a shimmer of heated air.
In combination, it may hint either at demon leaving his form or some kind of magi-chemical release. Given that Malagar is Valzira's pupil and knowing both what he's up to and that Valzira made Issa the way he is, its not hard to imagine such release being some kind of after-effect of the procedures done on Issa.
However, I think it's demon possession. Why? The whole scene is built to highlight those details, to hint at something. There is no narrative reason to hint at magical influence over Issa's form, since we know it already in most direct way possible - we were told he was modified by magic. If devs tried to hint at something, it must be something we did not know already, hence there would be no reason to
hint. So, we can reasonably discount it being magical effect. Possession, on the other hand, would explain bot the effects and strange plural when referring to self.
But that raises a question. FUCKHUEG one at that! Why demon?
IIRC, they are our natural allies - sun creatures, same as Dragon. I think, in very technical sense, Dragon kinda is a powerful demonic entity himself - or at least someone related.
Why would a demon plot against Dragon? Or did he?..
Valzira tells us he's about to do something catastrophically bad. Valzira is, amongst other thing, moon being, the opposite side of this divine conflict. "Something catastrophically bad" in her eyes may mean a lot of things - and not necessarily: "he's plotting against you!". It may as well be: "he's going to open hellgates!" or something.
Or even if he is plotting against you, what would his motives be? He seems possessive, which begs into suspicion a possibility of unrequited affection. What if some kind of demon got all drooly about his scaled cousin and went yandere-mode?
Honestly, I am at a loss of suggestions.