Hi GBKevin,
Wow - I find your question extremely interesting. Common wisdom has it that Google understands that '&' and 'and' are equivalents in Local, but that it's a wise idea to stick to one version or the other in your citation building. Your client's scenario appears to indicate that Google is differentiating between the ampersand and the '&', and that is fascinating. So, basically, it appears that Google is only considering the business as being onebox/knowledge graph-worthy if you are searching for the exact name version that you've provided in your citations, website, etc. (Rosenberg & Dalgren). I must tell you that this is the first instance I have a ever encountered of this issue and don't have ready advice to give because of that.
It would be interesting to test whether editing all citations would reverse the situation - in other words, that the client would get a onebox/knowledge graph display for the 'and' version and lose it for the ampersand version. That could be the outcome, but I can't be sure. If Google is being that sensitive about an ampersand, some questions that come to my mind relate to whether the change to 'and' would require re-verification of the Google+ Local listing (name changes often do) and whether the client would temporarily lose rankings or reviews if you made these changes. Again, I'm not sure about this, because I've just not seen it come up before.
I've done some searching in the Google and Your Business Forum to see if I can find any reports of scenarios like yours and am coming up empty-handed. I'm going to ask around a bit in the Local SEO community to see if anyone I know has encountered a similar issue. Sorry not to have clear advice on this, but there's always something new to learn.
Finally, it's perfectly fine to use designations like LLC, PC, etc., if these are part of your legal business name. Google is cool with this. What they don't like is if a business name as too many capital letters in it (i.e. BEST PLUMBER).
If I do get a response from any of my colleagues, I'll return to this thread. If not, I'd recommend you consider posting about this in the Google and Your Business Forum to see if you get any nibbles.