How do I disable the Armageddon spell, Might and Magic, and learn all spells in Astral with the Graal?

You can disable obelisks and, consequently, the Grail.

It seems that even without obelisks, the Grail still exists. You just have to search the entire map to find it. This was definitely the case in one of the versions, but it's been too long since I checked, so I don't remember.

P.S.: Armageddon cannot appear in the pyramids in SOD!

Have you checked?

It definitely happens in TE, and I don't remember seeing this point in the list of differences from SOD.