The Sven church really wants to ruin the relations between the two countries entirely if they are planning to assassinate their own prince on Leberis's soil. That would pretty much equal to spitting on the face of the Leberis leadership entirely. Do they want an open war or something? Such a church of evil, yet they still have the audacity to pretend to be a good faction, performing healing miracles and everything.
If they really do assassinate the prince there, Leberis should forcefully split the Leberis church from the main church. Many a regular believer wouldn't really care in the long run, as long as they still get their services. That's what happened in the Protestant Reformation in Europe. Some would move to Svendyardvania, but so what? Maybe some disgusted by the church of evil in Svendyardvania would move to Leberis in return, where a reformation would clean the religion of the deep corruption.