My 2 cents:

The issue of "how did he kill the king and queen and ascend" can be answered with power or subterfuge, or both. Perhaps he gained the permanent command of an orthon, the orthon assassinated the royal guard, court wizard, etc. He then used mortal means to kill the king and queen. Nobody knows about the orthon, that can be a surprise for the players. As far as anyone is concerned the old king's defenses were weak and the new king just took it because he was better.

The issue of "what do the devils get" and "how does he summon devils", I believe can be answered with the same question. The devils are promised that they get ownership of his heir. But rather than merely condemning his heir, his heir is a cambion who also serves as a channel between the planes. The devils want the kingdom, they're perfectly happy to let the new king live a long good life and then die of old age, only for the heir to take over and rule in the devils' favor once he is dead.