"I think Thorne would call all of the original Summoned who fled to the Free Cities and Solvunn problems. If they could have sent them all back, they would have."

That is why Istredd's suspected from the beginning they don't have the ability to do that. Which does seem strange, spells usually do have a way to reverse, but he has a low opinion on Thorne's mages, in truth. Many of their decisions seem foolish to him and they seem to be playing with forces they don't entirely understand. He can relate, honestly, both personally and through the Brotherhood, but it doesn't excuse their reckless magic.

"It would have helped them restart this experiment of theirs without anyone else talking poorly about their actions in the start." That seems the most logical thing to him, if he were running this show. Istredd knows that Thorne is dangerous and an active problem, but he has to live with them, so living within their control is all he can do.