No, do not apply WD40. Cameras use a special grease.
I would suggest you beg steal or borrow another camera and use that, even your phone camera is better than one that you know doesn't work. What about hiring one, there was someone on here did hires, cant remember the user name now though.
Film camera are dirt cheap
second hand, they go for for a few quid these days with a lens.
I really don't see the point of you going if your the photographer and the camera is having some sort of issue.
Contact the Bride and Groom mayby they can scrounge a camera from somewhere, someone kindly offered a lend on this thread.