For anyone else who might encounter this in the future: get in the elevator and press the button. As soon as you're slammed into the ceiling pres tilde (~) to open the command console. Type "noclip" (without quotations) to turn on no clipping. Wait until you see the elevator stop decending. Now use whatever is your designated crouch key to float down into the elevator. When you're at about the proper level to be standing, of at least below the ceiling, go back to the console by pressing tilde (~), and again type "noclip" to turn off clipping. The game should now sense that you're standing in the elevator, now fully decended, and the next level should load.