Is there any lore reason given for why there are no Divine Intervention spells in Oblivion? It would seem like Cyrodiil above all other places should have the spell.
There weren't really reasons given that the spells existed in Morrowind either though, outside of game mechanics. The character chooses when to cast the spell, which is just about the opposite of what an act of God is. Just a bad name for it though.
Is that sarcasm or does the Mage's Guild actually have magical jurisdiction over the Imperial Cult?
It depends on the relationship of the local Patriarch to the nearest guildmaster.
Funny...I don't actually remember any Imperial Cult shrines at all in Cyrodill...besides that one in the Imperial City. Unless you count those deserted ruined "shrines" in the middle of nowhere.
The Imperial Cult doesn't exist outside of Morrowind. It would be strange if you
did find one of their shrines anywhere else.