There would be some hefty consequences for Stephen willingly surrendering a casket to an agent of the Free Cities if Thorne found out about it, yes. If you decide to go this route, we suggest making sure it looks like there was a fight at the very least— the last place Ambrose and the mages of Thorne want any of their caskets going is the Free Cities, as they believe their enemies there would use the energy to create weapons of war to strike against the Singularity or even Thorne itself. Stephen would have been warned about this specifically before being sent on the mission.