First of all: Nice map, I dig it.
Notice the "operative" words (highlighted). On Onaviu, there is one huge land mass, with about four small island continents. Three are close to the major land mass, while one is literally half the world away.
There still doesn't seem to be a reason for the disease to have stayed on that one island for so long. Back when it was a problem for those people, as it was spreading across the island, it would have been carried by one of the several types of birds that migrate
thousands of miles. (We have them, they do it.) It would have gone to other parts of the world, but likely, once in an area, wouldn't get much further - though the birds could/would make it a world-wide problem. (BTW: Wouldn't your -100 modifier just make the RR a push? By that I mean, the elf would only have his stat bonuses as the mod for the RR. Right? Elves aren't immune to diseases, just "virtually" immune - unless you made them that way. The +100 is to reflect that high resistance, but still makes it possible for them to get sick. I can definitely see where he could become a carrier though.)
The assumption of the Demik going pandemic is based on the that no other peoples have been encountered since the Hatharnd have been working to resurrect the humans and dwarfs.
I don't think I understand this statement. Are you saying that people who first had the disease - and still do, but only in carrier form - are trying to bring the extinct species of humanity and dwarves back? Not sure what that has to do with anything about how the disease went pandemic from a single carrier.
Not necessarily if there are carriers who do not suffer the effects. They would need to be widespread or you would get isolated areas that the disease did not reach.
Except, it was so effective/powerful (-100 RR) that other than elves, the likely-hood of there being lots of carriers is very, very remote, and it was so fast acting and deadly it would burn itself out. Though, as it seems to affect just about everything it could have spread from nearly every creature to every other creature. but all of that just says to me, it is better served as being used as a "world-killer" much like a huge comet.
With a disease like the one described, this is I feel is the more likely scenario: Hundreds of thousands, if not millions of years ago, all life (except for maybe plants and insects) on the planet was killed off by this super bug. Leaving behind nothing but the structures the intelligent races built, most of which have been reclaimed by the lands. Now, the bug waits in the frozen carcasses of several victims in the high mountains where the snow never melts, waiting for another chance. And perhaps in caverns deep within the earth.
It just seems to me that this disease concept has fallen into the same situation that many writers and movie-makers have fallen. By wanting it to be uber-dramatic, it was made too nasty. So nasty that it wouldn't work the way it was said to work - unless the "perfect storm" of ingredients occurred, which just starts to feel too forced. I think, that it would have worked much better if the disease had a long incubation period: The initial exposure is like having a mild flu or cold which goes away after a few days. Now is when the disease really starts to spread through the body, and the populous as this is its most contagious time. A few weeks, perhaps a month later, and the person really starts to get sick as the disease kicks into high gear. by then, the disease has out-ran itself, people have taken it all over, so that by the time it starts to kill, it is too late to warn the next nation as many of them are already infected. Remember, a diseases function, as a living organism, is to propagate itself; if it does so too quickly with the results of killing its victims, then it runs the risk of killing itself off along with them. (As has happened.)
But it is fantasy after all.
As was mentioned. Again, all of the above can be made moot by the disease being said to be magical. Magic, is well, magic and rational explanations don't apply.
