Mr Light wrote:
As for him not remembering, I believe Alpert said only that he wouldn't remember the past day or something like that, not that he would forget the three years that LaFleur was in charge.
I think Ben does remember. Or he didn't remember (because his memory was wiped) but he ultimately pieced it together himself after 815 crashed, because he's seen that picture from 1977 before. There's no reason to believe that his "surprise" about the 815ers in Dharma was genuine, because Ben lies about everything.
He lied to Jack about not killing Locke. He lied to Caesar about not killing Locke, and not knowing who he was. He either lied to Locke about expecting Locke to come back to life, or he lied to Sun about not expecting Locke to come back to life. And those are just his lies in the last seven episodes! Why should we believe that he really didn't know about the 815ers in Dharma?