Holy wall of text, Batman!

1) Maybe. It depends on whether you, as the DM, thinks that this is an earnest effort, and that it fits. Perhaps they need to place it close to where the ritual elements and focus for the mythal is, to maximise the bomb's effects. You can't just lob it at a tower and assume it works, at least.

2) Whichever would you prefer? Most of your suggestions are good ones - I particularly like the Weave getting messed up (perhaps triggering Wild Magic surges a lot), and maybe throwing someone or something into a random spot. Or perhaps the opposite - fetching something from the planes that then can go on a rampage.

3) If the Wizard manages to execute this properly, maybe the Wizard gets to be the Magister for a while, to atone for damaging the mythal. Perhaps the Wizard becomes an avatar of Mythrien Sarath if the Wizard's elven.