They did. Where do you think all the Spider Daedra, Daedroth and Ogrim all came from?
They came from the Deadlands like the creatures of Mehrunes Dagon. While I'd also like to think they were an attempt to include the other Princes into the story, I doubt it. Oblivion is a pretty straight forward game and most of the information content is served on the plate, requiring little or no extrapolation by the player. If there was a serious intent to imply the interest of other Princes in the main plot through the presence of their servants, it's likely we would have been notified about it through books and dialogue. I think the creatures you mention were included because they are "cool".
I was gonna say that
The Ogres in OB are not Daedra, just regular Creatures.
The Ogrim are lesser daedra in service of Malacath. They didn't appear in Oblivion, but that has nothing to do with the Ogres, who are, as you said, ordinary monsters.
I was under the impression Camoran could wear the amulet as he was from Valenwood Royalty?
My understanding was always that Mancar's father, Camoran Usurper, was called "The Usurper" because he usurped the reign of the Camoran dynasty. I might be wrong, though.
In any case, his ability to wear the Amulet has nothing to do with his bloodline. It's due to the fact that through the story of Oblivion, he and Martin are reenacting the ancient conflict between The Rebel and The King, echoing the relationship of Lorkhan and Akatosh and also of Zurin Arctus and Tiber Septim. My interpretation takes this down to the idea that, from a mythological standpoint, they are both equally fit to wear the Amulet and wield its power... in other words, to rule.
Same with basically the whole main quest. We do all this because we need to relight the dragonfires, but a decent explanation on why exactly, is never given.
The Dragonfires symbolize the covenant between Men and Aedra ? in simplified terms, while Men worship Aedra, they will be protected from Daedra. Worship of Aedra is instituted through having an Emperor. No Emperor ? no Dragonfires ? no covenant ? no protection. The quest isn't so much to relight the Dragonfires, but to install a new Emperor and renew the covenant.