Doing my daughters tall three tier wedding cake, she wants it covered in Betty Crocker vanilla icing with edible pearls as decoration would Betty Crocker buttercream hold up to being in a marquee for eight hours before cutting?

Maybe. If I was using a canned icing I’d doctor with butter. Because I’m not 100% sure the canned icing.  Jonathan Joshua Russell had a video of doctoring canned icing. You could always experiment. Make a cake, ice with canned icing. Let sit in the coolest area of your house. See how it holds up.