I say do the litter box idea, that is great! The second cake idea would be a lot of work for something you wouldn't want to really eat. The litter box cake would let you have your laugh and eat it too! Great idea!
Yeah, that is insane! The MOB is already the 2nd most stressed out role (sometimes the 1st). Did you tell her that if you were her you would have it done? She probably wouldn't listen anyhow, but wow, think of the stress!
The reason why we post them is to share ideas, is it not? It is nice to be mentioned, but I don't think you have to, no 2 cakes are exactly alike. I love the fact that we can come on here and share ideas, and learn some wisdom from others whom are more advanced than us. That is very sweet of you to ask that question though.
You could do a sheet cake with a princess and Spiderman interacting. Maybe they could both be dancing, or wearing a party hat, or both carying presents or the number of the age of each child in their hand, like they are coming to the party too. I think that would be a fun and more simple way to do this. Good luck! We are looking forwward to seeing what you decide to do.