Hey David,
Man, you certainly have your ducks in a row. I have a couple questions for you to consider. It's kinda weird dealing with Google Places as so much is so inconsistent sometimes. It can be so many little things that impact rankings.
How old are your citations? If you have competition that had tons of citations for a long time, and your client recently got theirs; it won't matter how many you have for just a bit. There's going to be a small element of waiting. Give it a month, see what happens.
How does you client's location relate to the competition? If they are all closer to the city center, and your client is much farther, that's usually the culprit. Then, you're going to have to over-accommodate a bit (which it actually sounds like you're doing, so good on ya).
Generally speaking, the shared space shouldn't hold you back. I say shouldn't because by design Google should be showing "locations at this address" depending on your search. If you have sufficient citations to confirm your address, address on the site, etc. then you should be ok. You can test it though.
Look at a few of the other businesses with the same address and search for their keywords (just like a....GASP...common user...yuck). See how they perform compared to their competition. It's a lot of digging, but if you see that there are places at that address that do indeed perform, then you know that it should be ok.
If you only see one listing that performs, and all the others do not, then you might have some issues. But from the way you describe it, you're already "performing" on some level, so I think you should be ok.
Let me know if my thoughts so far aren't it. I wanna hear more.