1- No, we don’t say that all others will go to Jahannam. Those who see the truth, know the truth and stubbornly reject the truth deserve to go to Jahannam. But those who didn’t really see the truth and didn’t really know will either be forgiven by Allah or retested. And good Sunni Muslims who didn’t really see the truth and didn’t have any hatred for the Ahlulbayt (a) will receive the Shafa’a of the Prophet (s).
2- Yes, the Shafa’a of the Prophet can include them.
3- What is meant is that those other 72 sects are wrong and deviant. It doesn’t mean that all their followers go to Jahannam, as some of the followers may be completely ignorant. Yes, we should teach our children that only our sect is on the right path and other sects have deviated from the pure path of Allah.
4- You can believe that if they stubbornly reject the truth after seeing it then they deserve to go to Jahannam then this is ok. But to believe that all of them will definitely go to hell regardless of their circumstances is not ok.