You can try reading through the 5e players handbook or googling whatever you aren’t understanding followed by 5e. For example “spellcasting 5e” will explain spell slots, ritual spells, learning new spells, and preparing spells in the easiest way they can. 5e means 5th edition and is the version of DND the game is based on.
Some other quick tips, most classes know a lot of spells but can’t prepare them all, you can change your prepared spells at any time outside combat. A ritual spell can be cast for free without using a spell slot outside of combat.
Hmm, I don’t mind your idea at all but it is hard to implement so I see your trouble. Some folks have given good options but I’ll offer a different way to do it. I’d have one attack per player since it is a god, let him wreck house. Maybe increase the number of enemies to accommodate the attacks. Since this is a god regaining their power and their trying to control it I’d have two DCs for their check each round. First DC is if they listen and should be passed most of the time maybe 10 or 15. If they don’t then it might be bad for the party but otherwise they attack the target as intended. The second DC should be failed most of the time, maybe 20 or 25. If they fail this DC the the god still acts as ordered but releases a chaos burst as it does. I personally always use the d10000 list of chaos burst, its fun.