There's no danger by this point... consider it a safe point. If you didn't finish the absinthe, you don't have to, but you might need the alcohol. Either way, take the spoon and put it in the keyhole of the geen-lit portal's doorknob. It will fit perfectly, and reach the end of the keyhole with a resounding click.
Inside is a small elevator, with the most beautiful woman any mortal eyes can imagine, bathed in the green glow in just such an angle that the light refracts beyond her into the shape of wings.
The Green Fairy herself will ask you, "Going up?", and considering all the trouble you went through, it would only make sense to say yes.
Now, you have one more hurdle to clear. She will ask you, as you cross the line from the bar to the compartent, "How would you compare Beauchamp's surrealism to that of, say, Rene Magritte?" For your reply, you must say, "I've come to see more than art tonight."
I you don't, the green floodlight will blow out, the doors will slam shut, and the elevator will plummet through a seemingly infinite blackness before a read light grows brighter as the elevator nears the very depths of Hell.
Now, if your elevator begins to go up, the green light will also fade, but in its place will be the cool glow of the moon. But, before you even recognize it, the elevator will reach the top of its... well, let's call it a shaft to not get too intricate.
Now, I'm not as sure about this as the rest, but I've heard that, if the Green Fairy kisses you on the cheek as she leaves the elevator, you will always be blessed with a creative inspiration: a permanent, ever-changing muse. You can't ask her, you can't kiss her; she has to do it of her own volition. If not... well, nothing, but no reason to do it anyway and anger the woman who is responsible for keeping the Beauchamp paintings safe for so many years.