... What we saw of him in SmF as one of the Fallen who dominates the host and was pretty firmly in Tessa's camp seems contradictory to later WoJ that he's basically the magic nerd and a bit of a loner from the other Fallen who mostly just wants to refine his experiments ...
I think sometimes the Fallen end up with a largely-useless host, from their own POV. I suppose that might be just a mistake/mis-estimation on their part, or ignorance (being essentially blind and deaf in their coins, possibly unable to discern the qualities of their potential host).
These hosts can't help the Fallen in any way, advance their agenda, etc; instead they can only drag them down.
When we first met Thorny, he may have been burdened with such a host, and gone "fuck it, just gonna need to meatpuppet this one." Thorny may simply have found a more-apt partnership with Marcone.
Similarly, I suspect that alliances amongst the Fallen are kind of fluid and context-dependent; Thorny (I suspect) wasn't "firmly" in Tessa's camp, he was there as a matter of convenience or opportunity, maybe for a specific project with a shared agenda, etc. He may even have been a
spy in Tessa's camp, passing info to another of the Fallen.