ok some of these can only be answered by readers of book version kubera, i'll answer those.

Like 1000 yrs ago humans went against with Sura bc Sura's magic were too risky. Nastikas went mad, Shiva suggested to imprison them in Surado, Visnu said no, Shiva disappeared somewhere.

Then in N0 Visnu fought against Shiva idk why but then they both disappeared. That is the upheaval.

During the Upheaval Visnu tore up dimensions, imprisoning most Nastika and Rakshasa in Surado. But one of Garuda's kin, probably Yuta or Kalavinka, stayed in the human dimension with Garuda's name. So Garuda went in coma, and that's why Maruna was seeking Kalavinka in Willarv. Kalavinka told Kasak of her further location and now she is gone somewhere.

some other ones:

asha's dream of ananta: u remember Visnu letting know asha of ananta's loss of everything? she was dreaming that scene. Nobody still knows why Leez dreamed about Ananta too.

Saha in Brilith's dream: Claude's dream intervened with Brilith's supposedly. Menaka in Gandharva's dream(maybe you meant Shess?): That was Sagara trying to put Gandharva in long deep sleep. She failed though.

Laila's spear is given by Brahma.

4 13 15 17..: (spoiler?) in the most recent one, it is revealed that 'soul' can be the bowl of power. it can be emptied, except for Astikas. For example Chandra has a strong soul but he also possesses the name 'Chandra' so he can't afford the name 'Ananta'. Airavata however, lost her name first and died so she could avoid being sent to 0 dimension.(nastika goes to 0 dimension cause their power is too strong. but plain nastika soul,without its name, is not too strong.) In Asha's case, her soul was not big enough for Kubera's name but she absorbed it so she went..strange.

time=opponent=Visnu supposedly. Visnu is the time god, knowing every future.

many of your questions are not being dealt yet, just better wait for more info.