No they didn't, well maybe a lot of Nords did, but not everybody. There have dragons around during Tiber his time, a few others have been confirmed at later dates. You are right that many once believed dead are now returning, but this is still a different situation. Dragons can only be killed by the Dragonborn, all those that weren't can come back. The Dwemer have nothing that symbolizes that.

Also the dragons are the harbingers of the end times according to many a Nord. That means they knew the dragons would eventually come back.

I didn't think Nirn was a planet, but a plain of Mundus, the Mortal plain of existance.

If I remember right, lore dictates that Nirn is flat and the planet only has one side. (Like the Earth, before 1492. )

Nirn is the central planet of the Mundus, the hub of the Wheel.
It is globe shaped, but similar to how we used to think about our planet, it is a geocentric universe, meaning that Nirn is in the centre with all things orbiting around it.

Regarding the Dwemer, I recently invented a theory about their dissapearance.
Its a little complicated and involves timetravel so bear with me.
It also is a heremetic theory, meaning that by its nature it cannot be proven or disproven, its just a bit of fun.

"What if Numidium did come to life, only moving backwards into time and not forwards like everyone else?
I cannot remember if Numidium went explodey at the time of the ascension of the Triune, but if it did, an explosion viewed backwards is coalescion.
Now the reason that no-one saw Numidium stomping around before it got activated is that in the then-then, the first past, where they walked to the future point of activation, it did not because it was not active yet.
Only in the now-then after the activation does it move towards the beginning of time.
Numidium was supposed to return to the 'first brushstroke of Anu and Padomay' so timetravel seems logical.
If the causality was reversed and the Dwemer started a backwards journey in time the moment they became one with the robot, then the Dwemer souls do not inhabit the Numidium in the future-now past the point of Kagrenac's actions and its logical it didnt come to life.

In short, could it be that the Dwemer are on a journey travelling backwards in time, unseen by anyone because in the then-then it had not happened yet, only in the new now-then the Dwemer travel in.

I hope this makes sense. As Pratchett said: 'Its hard to talk about time travel in a language evolved to tell other monkeys where the ripe fruit is.' "

Don't think that makes to much sense. We know the Numidian can travel through time, in the sense that it's fighting Altmer mages well into the fifth era even though they surrendered to Tiber Septim in the third era, this seems to suggest that time travel has immediate and noticable results. If the Dwemer were to become a god in an earlier time then most likely Akatosh would break and there simply would never have been any Dwemer to begin with.

They havent arrived yet, they are still walking through the era's at a rate of one year per year.
But as I said, its a heremetic theory and by its nature cannot be proven or disproven.
Though it would neatly explain why Numidium did not come to life.
It did, only compared to everyone else with reversed causality.

...Have you played Morrowind, or even explored it via UESP?

That's more or less what I said, except for the god bit. The Elnofey were created on another plane. They were moved to this one and begat people in the new plane (Nirn). There is a big difference than having your origins as a race on a physical plane (Nirn) if you think about it. Are descendents of aliens, aliens? Or do they become something else? If they refuse to accept their dwelling place as home and want to go back to their "proper" dwelling place, as some of the Mer do, do they even accept they are from Nirn? Or Apart from it? (alien)

My Theory: The Dwemer had possession of an Elder Scroll, supposedly the very same on that the Ancinet Nords from the Merethic Era used to send Alduin through time. Maybe they too sent themselves through time. It would be a very interesting DLC, something having to do with Calcelmo discovering what happened to the Dwemer.

I would actually enjoy that lol

Good lord this gets confusing. All I wanted was aliens! Not golden skin...

No.

Eric Von Daniken appeared in the TES universe and explained to the Dwemer how religion actually originated from E.T.s

So they built a space ship and are currently traveling at relativistic speeds towards the nearest solar system?

Thats what you get when you go TES.
Wait till you learn about Amaranth.

Dwemer you say? ALIENS! - Giorgio A. Tsoukalos