At 9PM you'd still probably be ok, as I think most stores close at 9:30 PM. And there are some restaurants there too, including Legal Seafood, so I'd imagine that one could still walk through the mall until at least 11PM. After that though things could get dicey with trying to get through the mall.Thanks for all the information. I'll take this post with me (though I wonder about access to the mall around 9 p.m., when I think the train is due; while most stores are probably closed, there might be some restaurants or other places open). I'll have printed directions with me and the hotel phone number. On top of that, the Hilton is probably fairly easy to spot from outside the station if I'm having trouble getting my bearings.
Thanks again for all the assistance.
As for spotting the hotel, there are lots of tall buildings in the Back Bay/Copley Square area, so I wouldn't want to bet on spotting the Hilton from the station. Carrying a good map from say Google or Bing, or a SmartPhone that can pull up street maps would be most helpful, just in case you can't get through the mall.