Cassandra kept insisting that she wasn't Angelique to Barnabas because she didn't want him to know who she really is. Rather weak reasoning on her part, because she didn't fool him one bit...still she had nothing to lose by denying her true identity.
I think the reason may be that she figured that if she conversed with Barnabas as Angelique, she would be tempted not to re-impose the curse after all. This would indicate a self-knowledge that would be atypical of Angelique, but let it pass.
(2) I thought the portrait was there at Collinwood to put Roger under Cass and/or Ang's spell. *** I suppose we don't know for a fact that Ang had the portrait in her possession before it was at Collinwood. My memory is worse lately though. I'm not sure what I'm getting at, either.
I never had any feeling that the portrait was essential to Roger believing he was in love with Cassandra - perhaps because of the drug that Cassandra slipped him on their wedding night. But we were not allowed to see the scene When Roger Met Cassandra, so we don't know exactly what happened. It happened at Stokes's place - right under his very nose, apparently - so it must have been interesting. Maybe someday we'll get a Dark Shadows Deleted Scenes DVD with that on it.
As for the portrait, we later learn - if I remember correctly - that
[spoiler]Nicholas sold it to the antique shop from which Vicky bought it[/spoiler]
so its presence at Collinwood clearly was essential at that point.
By the way, I love the new spoiler feature. The old one died so long ago that I can't remember it exactly, but it seems to me that the new one is set up much better than the old one was.