Was talked into icing wedding cake for this weekend in IMBC... Problem is I only work with fondant. Should I just ice in my crusting buttercream dream? Will it be ok in the summer dry San Diego heat (inside in ac)? but using imbc if it starts refridgerated and then sits out in ac for 5 or so hours what happens to the consistency? I've made it a million times before and I know when it goes cold to room temp it separates...

It will only separate if you mix it when it's cold. Just sitting (in a bowl or on a cake) it should be completely fine. IMBC is the most stable of all the MBCs. If the room is reliably a/c'd and it is not in direct sun it should be fine. If you promised the customer an IMBC cake, you should not switch to a different icing without their knowledge.

I should have clarified: they did a tasting with imbc as a filling which I always use. They just switched from fondant to bc and so they still has it as a filling but we never discussed what the outside of the cake would be iced in. I agree integrity is is incredibly important. Thank u so much for the help, it's just in time, I'm about to ice!