US Immigration from Canada

My fianc and I are about to send off our petition probably sometime next week, as we've been busy compiling everything.
I do have a few questions though regarding not just this petition but the process to ensure I am ready to get going.

I was told they ask for vaccine records, but what do you do in the case that you don't know/have the yellow paper?
And are there any vaccines that are mandatory that i'd get rejected for not having?

Are there specific guidelines on the wedding for the K1, or is it just what the laws are in the state?

Is there any way to have a 'backup sponsor'? I would love my fianc too, but in the event for whatever reason we get rejected for that, am I able to give another form of a different sponsor or would we need to start over from the start? (Cr-1)