Thanks for the quotes but I can't see where they believe that they will know that this final sealing has taken place (before the GT begins). From what is said in the quotes, they would only know because they have witnessed the destruction of Babylon the Great which they say occurs after the GT begins.
I am very curious about this because if they are saying that they will know that the final sealing has taken place then they will also know that the GT is imminent and if they are teaching this then they are telling the flock that they will have foreknowledge. This foreknowledge will be unprovable. The flock will just have to take their word for it because there will be no physical evidence.