The thing with the bomb is that the timeline doesn't really work out as Oda has set it now. Because Big Mom is planning to open the box after the ceremony. But Pudding is going to shoot Sanji when they're about to kiss. So the SH's have to act during the wedding. Which means that the bomb is still in the box.Well rationally speaking the bomb shouldn't be that powerful as they were keeping that box inside Ryougou Palace and I find it hard to imagine they would have done that if there was a risk of damaging the palace severely, though for the sake of the story Oda might ignore that.
Personally however I believe the bomb is not meant to inflict any significant damage, but to set the defining tone of the situation. Don't forget Dogtooth's Observation Haki. If he realizes there's a bomb inside, this distraction might open up a chance to destroy Caramel's picture or alternatively if he realizes someone is going to try to destroy the picture, this might lead to him being unable to stop the bomb. Actually now that I'm writing this I started to think that the primary purpose of that bomb might be to destroy that picture, which then would result in BM throwing a tantrum.
Either way no one is expecting the bomb, so the side that adapts the quickest to the situation, will gain the advantage.
Maybe one of the high guests who seem awfully interested in the box might sneak a peak in before the ceremony ??