It makes sense to me that the croyel contributes something besides possession.
We also have to wonder about why croyel, and not Raver, I think.
This MAY have something to do with the form of Jeremiah's blankness. If it was created as some sort of means to defend himself against Ravers, it could explain the croyel.
As for Jeremiah's loyalty: I think Donaldson wants us to believe that it's up in the air, and that Linden needs to consider that it is up in the air.
But I don't expect Jeremiah to be happy about being possessed.
Then again, we have this:
Still, I think that this is misdirection.In [u]Fatal Revenant[/u] was wrote:"He's Lord Foul's prisoner," she replied through her teeth. Tell her that I have her son. "I've known that since I first arrived. One of the croyel has him now, but that doesn't change anything."
"The Mandoubt does not speak of this. Rather she observes that a-Jeroth's mark was placed upon the boy when he was yet a small child, as the lady recalls."
Her statement stuck Linden's heart like iron on stone; struck and shed sparks.
The bonfire, she thought in sudden anguish. Jeremiah's hand. He had been in Lord Foul's power then, hypnotized by eyes like fangs in the savage flames; betrayed by his natural mother. He had borne the cost ever since. And when his raceway construct freed him to visit the Land, he may have felt the Despiser's influence, directly or indirectly. The Mandoubt seemed to suggest that Jeremiah had formed a willing partnership with the croyel. That his sufferings had distorted and corrupted him within the secrecy of his dissociation.