Maybe the intel part is more to officially seal their status as traitors to the Empire, less to actually gain valuable information. I'm sure any interrogator will notice in little time they are too idiotic to know anything useful. Charlotte never cared about anybody else but her dead mother anyway, and I doubt the other nation will be interested in anybody's dead mom.

In the end they are going to be tools for propaganda if anything. Should a war occur and the other nation win, they could marry Charlotte to their own second prince and place them to rule over the former Empire. It could grant a bit more validity to win over some subjects, perhaps.