1. Can be learned it from previous Kais or even he might've tested it on weaklings who're not strong as Goku and Vegeta.
2. They didn't unfuse because they were swallowed while Gogeta created barriers to be prevented being swallowed. At that time, indeed it was supposed to be Boo's auro related
And time limit is introduced here, but it didn't break anything. We still can argue like their limit just wore off inside Boo. And they only assumed it was because of Boo.