Okay,so maybe ascension doesn't cause a true dragon break,but is Mannimarco becoming the God of worms the initial reason the Warp in the West started?
No, that was just 1 outcome of the Warp of the West. The WotW would still have happened, even without Mannimarco becoming a god. The whole Warp of the West came because the Numidium was reactivated and, somehow, 8 different groups got a hold of a Numidium at the exact same time, when there is really only supposed to be 1. You have the orsimer, Sentinel, Wayrest, and Daggerfall who used it to defeat all the kingdom around them for power and land, The Blades to subdue the kingdoms, Mannimarco to become a god from that point on (all points before are still him being a mortal and a lich), the Underking to rejoin with his soul and finally die, and the player, who gets stepped on. If all events were to happen as such, it'd be a giant cluster poop. Because of that, Akatosh broke and things became screwy (8 Numidiums when there should only be 1). To fix this, the Jills took some duct tape and glue to patch up ol' Akatosh there, and made the events end with all groups receiving Giant Stompy Bot, but those kingdoms who had it were not destroyed and incorporated those that were to their lands, the orsimer got some of their own land carved out, the kingdoms were subdued to a peace, the Underking got his soul back, Mannimarco became a god, and the player was stepped on (but this you can have your own leeway if you so wish as there could have been 7 other agents).
Now, what you may be thinking is V-man. I think someone here said when the Tribunal became gods, Vivec changed it so that he was a god at all points in hisher life, making it so he was not mortal from all points before ascension (different from Mannimarco who remains mortal for all events prior to ascension). To do this, Vivec broke Akatosh, edited his past so that he was always a god, instead of a mortal becoming a god.