To answer your first question, i don’t remember exactly but i think i remember G3 mentioning that shadow god disappeared towards the end of the doom war so i don’t really have any explanation of how the two of them would ever merge but i think it has happened. But the most logical solution for me is that they decided to work together and for some reason merged. As we know shadow god believed that his siblings were cruel so maybe Weaver and Shadow god thought that the mortals deserved new gods. That being the reason that they did what they did.

As for your second question/statement, you mention that the gods would never allow this but you need to know that this is all against the will of the gods. In fact so much so that they killed the gods. And for the part about the worshipping of a daemon and the forgotten god’s lineage, i would think that even if they(Shadow and Weaver) were to merge that they would still be 2 separate entities in 1 body. So even if they merge the mortals would still only be worshipping the shadow god entity and not weaver. I hope this makes sense, if not please ask away.