It has been my experience that you will never get the squares back in time and correctly signed to have the quilt for the wedding. It just doesn't happen. The suggestion to give the quilt later is a good one. You might also consider:
Have the top made for the wedding but
Have guests sign on fabric for the backing instead of the individual squares for the front.
This worked well for me on a memory quilt for my mother. Didn't have to worry about guests writing outside the lines. They could also write more greeting or good wishes. And a minor point, she didn't have to look at signatures in the future of those who had died, separated or divorced.
The bride and groom can see what they will be getting and since the top will already have been made, you can have it ready amost by the time they get back from the honeymoon.