The story says all the heirs are dead. For sure and that Martin was the last. Was the rumour or a report on a piece of paper, or what Occato told you? Could someone have been lying to you or to others to protect an heir if the assassins murdered a double instead of the real guy and thus setting up something for the future? Maybe someone who did not understand about the amulet and stuff?
1999, the assassination of the emporer and his 3 sons was published in the black horse courier. Also, the dopplegangers mentioned in Morrowind were part of a rumor that: Jagar tharn had done away with the emporer's sons and replaced them with new individuals who magickally took the image of uriels brats. It is known that Jagar tharn took the image of Uriel septim, and fears that the emporers sons could really be Tharns drinking buddies in disguise arose after rumor of Uriel's poor health reached the public. If they really were dopplegangers, its likely that the real sons are either in the custody of Mehrunes Dagon, or dead.
Also, Uriel septim himself confirmed the death of his sons while advancing through the prison, and his ability for prophesy has been proven by the conversation that he had with one confused prisoner.
As to your criticism about the emporer's assassination and the methods the blades used to smuggle him out of the city:
If the emporer were to remain in the palace, He suffers increased risk of the staff and palace guards targeting him (as they could have been assassins). If He takes to the streets in order to leave the city he is bound to be killed, as their are multpile mythic dawn agents in the city and the entire population has the capability to cast a fireball.
He could be attacked from the streets, windows, and rooftops by magickal or mundane means. One need only look at the death of JFK to see how hard it would be for the emporer to make it through the streets if a well thought out assassination by a sufficently skilled marksman or magister were to be attempted. To put it simply, Without means of magickal transport the only chance he had would be to use a little known passageway in the prison system.
Lets not also forget that you have no gurantee the mage telporting you is going to teleport you to the right place. A guild guide could have sent Uriel to lake Arius.