Nah.

Real world wedding tradition has the Bride and Groom always eating the first slice before anyone else eats. There is no stretch to assume that in GoT the same would be the case, and especially at the wedding of the king, you can assume he would be the first to eat. And even if his bride eats first, Marg dying would not have a dissimilar effect in regards to chaos. I don't know why you think several minutes is needed. If it was the pie, the poison was very fast acting. You're inventing holes that aren't there.