they invite batman and kidnap bruce wayne so they’ll have two witnesses, bruce has to escape so they don’t think batman snubbed them
Consider: the Batfam realise that Bruce is missing the day of the wedding, so various members put on the batsuit so Batman can be present for the ceremony.
The guests end up being three different shades of batman, Bruce Wayne tied up with rope but one arm free so he can toast the couple, and an Elvis impersonator to officiate the wedding.
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