With the decline and everything, well, what are the implications? For both the deity Talos and their place in the world? Talos' effects on the world, will Shor/Shezzar replace them in the pantheon again? How does that effect Talos' mantling of Shor/Shezzar?

You got it backwards, Talos fills in the Missing God's place in the scheme of thing, and his "fall" so to speak would mean that the worship of Shor/Shezzar/Lorkhan would effectively be dead again. You know, they themselves being sorta kinda dead. It would just revert to how things were prior to Tiber Septim, which is fittingly happening across Tamriel as of now.

Weeeeell, there was a priestess of Lorkhan in Whiterun in the 3rd Pocket Guide as of Oblivion, trying to turn the cities luck around and into prosperity. What her deal was is anyone's guess.

Well yeah, I understand that. But there was worship of Shor/Shezzar before Tiber Septim, and the Talos oversoul mantled the dead god's place. But since worship and belief are so important, I just wondered what would happen, or has happened. Skyrim worships Talos, as they worship the Wulfharth part of the Talos oversoul (or the myth that he was a Nord that came from Atmora, again, Wulfharth, in the First Era), but also worshiped and still honor Shor.

A thing to consider is also a possible change to Shor's status. As in, Shor being dead. Red Mountain erupting, the Moons - his body, dissappearing for two years, only to be brought back by "Dawn magic". Maybe I'm just thinking out loud, but could the mantle of Shor be reclaimed from a weakened Talos? Also, what of the Underking(s) and their status?

Here's the problem: Talos worship is experiencing a resurgence because of the Thalmor's prohibition. It's an act of defiance against a faction that was a hated enemy within a generation ago.

As for the Void Nights... I doubt the Thalmor claims that they brought them back, and I'd be willing to bet they caused their disappearance to begin with. They have such a hate-on for Talos and Lorkhan that I think they tried to erase Lorkhan's physical remains from the sky, found out it was only temporary at best, calculated when they'd come back, and then paraded into Eleswyr saying "Tah-daa! We brought the Moons back!" when they finally did.

Red Mountain was caused by seismic disturbances from the Baar Dau impact that obliterated Vivec City agitating the mountain into erupting. The Heart of Lorkhan had been unbound and free from the Mountain for some time by then.

There's nothing to reclaim. A prerequisite to trying to become like Shor is to die, and thus your worship along with it. The Nord's might venerate Sovngard by extension, but its the land of the dead in the first place and thus has no bearing on that of the living. The Nord's would not worship a dead god the same way they would Kyne, Dibella, or Mara.

http://www.uesp.net/wiki/Skyrim:Song_of_the_Askelde_Men, but they did not venerate him as a god. Talos is unique in this aspect as he fills in the space Lorkhan left, whole, complete, and a being entirely his own then being an extension of the same story. Worshipping something does not mean you're magically counted among godhood.

^I'd have to agree with the majority of this.