As he's gone/left

1. The option "stall doors" is correct here. The "stalls" in a restroom are themselves the small rooms with toilets.
2. The word "as" can be used to link two verbs happening concurrently, like "when" or "while", but this pattern is used for verbs in the continuous aspect ("he leaves/goes"), not the perfect aspect ("he's gone"):
   • "As he sank into the bath, he thought about his day."
   • "As I eat my sandwich, Susan plays the piano."
   • "You will notice many beautiful buildings as you drive down Main Street."

In this case, you might use the present continuous "as he leaves" or "as he goes". If you mean that the feet appear after Ben leaves the restroom, then another phrase would be more appropriate: "As soon as he leaves", "after he's gone", etc.

1. A "stall" is the booth itself, not the door to the stall. So I'd suggest it should be "stall doors".

2. I prefer "once he's gone" (which would suggest Ben's left the restroom and is moving down the hall (or wherever)) or "as he leaves" (which suggests the feet appear just as Ben is passing back into the hall (or wherever) from the restroom). But also, feet wouldn't appear in a stall, unless they're some sort of magic item...I think that needs a re-word - maybe "a pair of feet become visible under the stall door" - I think it depends from what angle you want the reader to visualise what's going on. Also, I think "a pair of feet appear" sounds better than "a pair of feet appears" - that doesn't appear to be correct to me.