Garage Door Sensor Issue
UPDATE

After spending a few days trying to decipher the troubleshooting instructions Liftmaster sent me, I've resolved this issue.

http://liftmaster.custhelp.com/app/answers/detail/a_id/2715

As I mentioned originally, I could hold the button down on the wall control to "force" the door down. I did this and then waved something in front of the sensors. This reversed the door. So, the sensors are working fine. 

I ended up pressing the green "Smart" button on the back of the main unit and then pressing the remote button once. This seems to have re-added the remote to the system. Both remotes work normally now.