@Guardian Angel Haruki, honestly, the more I think about it, Daniel won't allow a shapeshifter who means them harm to live long enough to, say, go find the nearest settlement and use his shapeshifting to impersonate the party to frame them for crimes they didn't commit.
So he'd probably work with Malcer to ensure that Bartholomew dies as soon as possible.
That is, if Barth being a Changeling is obviously known.
You presume much about how "gung ho" Malcer would be to murder a party member, no matter how unempathetic he can be towards those that aren't his friends. Consider that, starting out, Malcer is likely at least slightly disassociating his situation in this new world. He might partially feel like he's in a dream, but it feels too real and too clear to think like that. So, for safety's sake, he operates with the assumption that everything is real and that his choices matter and can have consequences.
Regardless, on his stance on murder, this isn't exactly accurate without going more in-depth, but basically he sorta sees his party members as more "real" than the locals, who are naturally "NPCs", and he sort of treats his situation like that. Now, as an avid role-player, he has this understanding, a gentlemen's agreement of sorts, that one of the biggest cardinal sins is to deliberately sabotage the party or kill a fellow PC. For this reason, no matter his garbage attitude, Bartholomew has some leeway. As long as he, himself, does not commit that cardinal sin, Malcer may not like him, but for the practicality of greater firepower and utility, he is willing to tolerate him... to a degree.
And consider: Malcer may be unempathetic, but he's not a psycho/sociopath. The party to him are "real people", and killing them is nothing less than murder. Malcer was never a murderer in his past life. He was a pretty normal guy, and that hasn't changed, no matter how much he's now enabled to exercise his power fantasies. It's one thing if he's killing a bandit or monster that attacked him on the road; he won't hesitate in such a case. But to escalate to murder against someone that's "only" an asshole and hasn't tried to kill him or his friends first? Wouldn't be something he'd feel like would be "justifiable" to the party.
That said, if Malcer were to (perhaps inevitably) plot Bartholomew's murder, it's unlikely he would collaborate with any of the rest of the party. To spare the morals and feelings of people like Ciel and Finn, Malcer would likely go it alone. He would want Bartholomew's death to not be something that could be tracked back to him, because he cares about the rest of the party and wouldn't want to compromise his reputation in their eyes. So, if he commits premediated murder like this, he's not going to ask anyone else to stain their hands.
So, jokes aside, don't expect Malcer to do more than menace/threaten Bartholomew out the gate to keep him in line. Actually trying to kill him is a whole other level of response to be reserved for an appropriate time and offense(s).
I think Bartholomew would realize how invaluable being a Changeling is for him, and would most likely keep that secret.
I'm currently presuming that the party would arrive in their base states with no abilities active, given they are basically just growing into fresh bodies. So, I wonder if Bartholomew would even be able to hide it from the party... It's funny though, but even if he can't hide it from them, when I really think about it, it's still incredibly hard to defend against without straight-up mind-reading. It's pretty clear to me examining that ability why changelings are often feared and hated, because it's absolutely sinister what they can do unchecked.
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