There's a real world precedent for this in the way our own mythologies and world religions form and alter over time. Let's pick one of the less controversial examples - Hercules, the Roman/Greek culture hero (chosen because he's a prime example who is not worshiped by any living person, and thus safe for discussion here). Hercules is based on Heracles, whom in turn was an amalgam of many previous local culture heroes of Greek and proto-Greek city states and farming communities. What began as hundreds of exploits by dozens of men were eventually condensed into one almighty demigod. Very few or none of the original names or tales remain. We have Hercules, who himself was originally a separate man from Heracles but who absorbed all of his feats and tales as well. He ate him to become him. As do the pantheons of Tamriel eat lesser mythologies and et'Ada , absorbing their abilities and histories.
Perhaps this works not only on a metaphorical level but also a metaphysical one. Perhaps beings like Umaril's father literally cease to be altogether and really do have their traits absorbed by various other culture heroes and gods. Perhaps the Aedra, being part of Mundus itself, are susceptible to changes within the cultural attitudes and customs of its sentient races. The pantheons have been altered several times before reaching their current states, and may be altered several more times before the end.
Side note: As a potential future topic, I have observed the strangeness in the apparent fact that Aedra, who represent stasis, can reproduce with mortals and Daedra, who represent change, supposedly can not..this was challenged, however, when I remembered that Molag Bal has reproduced more than once. Is it possible that there is a bit of Aedra in every Daedra Prince, and vice versa? There would just about have to be to keep them from dissipating altogether. This would reinforce my hypothesis that every Daedra Lord uses a mortal person or the remains of one as a sort of focus or anchor to his own realm. If not a person than a piece of an earth bone, or some other creatia.