1- On the basis that one obeys what God wants or not. If someone has proper beliefs, fulfills the obligations, and repents from sins then he is entitled to go to paradise. Otherwise he is not entitled. If someone dies with the wrong belief, they are not entitled to paradise (though Allah may still forgive them out of his mercy).
2- Wilayah is a condition for paradise. We have many hadiths on that. If someone deliberately rejected Wilayah (knowing it is the truth) then they are entitled to Hell. But if someone was truly ignorant and didn’t really know, then Allah may forgive them even if they did not believe in Wilayah
3- Imamate is not a jurisprudential matter. It is a pillar in belief, and other sects are not obeying Allah by abandoning Wilayah. Furthermore, by not following the Ahlulbayt, they are not worshipping Allah like the Prophet. They don’t pray like he did, they don’t practice Islam like he did.
4- Yes one of the questions we are asked about in the grave is who is your Imam.