are all the towers located in Tamriel? or do some exist in the other Continents, like Akavir, Atmora, Yokuda, or Pyandonea?

The Orichalc tower is suspected to be on Yokuda. Whether it was destroyed during the cataclysm or not.

Also Talos and the Clockwork City seem to act as towers, and are more mythical than physical ones.

Other known towers are on Tamriel AFAIK. But we don't know them all. ESO added at least one new tower (Doomcrag), or maybe a second one (Aetherian Archive).

The Doomcrag eventually failed though, didn't it? The Coral Tower and the Pillar of Thras seem to also be related to other towers in some way.

Yep, the Doomcrag was incomplete, and utterly destroyed during ESO. But it somehow worked, as it was able to affect its surrounding environment.

And I forgotten the Coral Tower, thanks!

It's not implausible that other continents may have towers, or equivalent metaphysical landmarks. Even in planes of Oblivion we've seen the metaphysical mechanics of towers (such as the spire in Passwall and the Sigil Towers in the Deadlands, specifically). Wherever a civilization in the Aurbis exists with an interest and knowledge in tampering with or tweaking the mythic and metaphysical, things like towers may exist.

I strongly suspect the Coral Tower serves the same purpose for Pyandonea, but there's been nothing explicit on it.

Apparently, the http://www.uesp.net/wiki/Lore:Coral_Tower_%28Thras%29 was in Thras.

Let's not forget the Summit of Aporcrypha, which looks like a towr as well, and the supposed tower in Varmina's realm that houses all of the dreams/memories she collects.

If we're talking about Daedric realms, there's also Mephala's Spiral Skein, as well as copies of towers in Coldharbor.

Yeah, I'm always getting those two mixed up. This won't be the last time.