Yes, you can wear an essix retainer over an implant site. It might be a better temporary solution that an immediately loaded implant as there will be less force on the implant as it heals.
I would recommend an implant. #31 is already compromised. An implant has better longevity than a bridge.
It is generally not recommended to bridge an implant to a natural tooth. However, it is possible to attach 2 crowns to an implant... once you are not a grinder or clencher. Your Dr will make that decision based on your specific situation.
It is best to wait until after delivery for elective dental procedures. The bone graft will be stable and ready when you are able to proceed with treatment.
It is unlikely that decay under the crown of a root canaled tooth would cause the symptom you described. You should have the root canal result evaluated by an Endodontist to determine if all is well with the root canal procedure. Crack tooth syndrome is also a possibility.