As far as I could tell, the storyline went as follows:
The events in Myst V happen 5 years after the events of the DRC. That being said, I think it's safe to assume that there's no set timeline , only that the DRC stuff will happen, eventually leading to another fall, therefore Myst V being after the new Live is completely feesable. Plus, I forget where I read it, but it was said that Myst V will fit into this whole story, just not how we thought it would. Since the cavern looks pretty much the same from the window in K'veer in Myst V as it did in ABM, albeit a bit darker (that can happen in five years) I think it's safe to assume that Myst V is future.
As for Atrus' age: Remember, he's D'ni, therefore he can stay alive twice, maybe even three times as much as the oldest human, especially given what precautions me might take to keep himself healthy.
Now, about that DRC poster in Myst V:
I took it as the DRC coming back to work even more after they first left (the whole construct support in the Great Shaft kinda supports this theory even more), so I saw it as a little way of Cyan telling us that they wouldn't give up hope on Uru Live, even when it looked like they did.

At least, that's what helps me sleep at night.
-J-
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