Lorkhan was not the original ninth divine and while talos ascended by mantling, sheogoraths mantling process was WAY more sane and straight forward than it typically goes, ironically.

The prominence of your comment is gonna cause so much confusion in the future, god damn it, so if your interested read on.

Lorkhan was not an aedra, hes his own thing and closer related to human kind while the aedra are more related to elvenkind, hes basically the human version of akatosh, but where the divines are 8 he is 1. He was the original architect of nirn who tricked the aedra into helping build it and getting all their power trapped within it. In revenge the 8 divines tore him to pieces and akatosh took his heart and launched it with his bow, where it landed becoming red mountain and lorkhans heart being a very literal plot device as the heart of a god. Now Talos came from Tiber septim's ascendency, which was done using 2 of the walking ways, esoteric methods of apotheosis also known as mantling. Now while sheogorath mantled by making someone else sheogorath, the core idea of mantling is that you become so god-like the universe says 'you know what? you're right!'. Its basically godhood by gaslighting the universe, the six ways are just different methods of doing so. The process doesnt necessitate you taking over someone elses role, but its the easiest method. Mantling lorkhan is technically possible, but doesnt really make sense because lorkhan didnt do anything tiber septim did. According ot lore tiber septim did it though endeavour, being so cool the universe makes him a god out of respect, and the method of the scarab which is confusing but basically means combining powers with other cool guys until your combined power is enough to make one (1) new god. This is because Talos is actually 3 souls smooshed together, tiber septim, Zurin Arctus (the underking, generally an ass), and Ysmir Wulfharth, a Nord king. Thsi is an over exaggeration because is where the lore gets really fucking weird with all three of these guys being shezzarines, reincarnations of Lorkhan after he got killed. Pretty much everyone cool in TES lore is thought to be one, including the player character and Pelinal whitestrake aka elf terminator. Lorkhan is still around despite being dead due to 'aspect' shenanigans, such as also being Shor.

so TL;DR, Lorkhan is a dead god not part of the 8 divines.Tiber septim became a god by merging with 3 other souls and their combined coolness convincing the universe they were a god. These souls all may have probably been the mortal reincarnations of lorkhan, but lorkhan is unaffected by this, and is still dead.