JohnBoy...
There is an old saying...
"If it looks like a duck, smells like a duck and feels like a duck ... then its a duck!"
You say you have this feeling that she is just telling you what you want to hear. Then your intuition is probably right. Mine did the same to me and at one point she even admitted (post breakup) that a lot of the things she said were because she thought thats what I wanted to hear.
Thats called manipulation. If you are right in your suspicions, she is trying to get you in a place where she feels comfortable and has the control .... where you are thinking that you are comfortable in her beliefs, attitudes and behaviours. Because she has no real capacity for empathy, because she doesn't really even know who she is or what she believes in (apart from her own need for attention), she is mirroring back to you what she thinks you want to hear. Does that make sense?
There is nothing of an "adult relationship" in what she is trying to do here ... its all about control and manipulation for the greater goal ... attention, caregiving, sympathy ... whatever, as long as you are attending to her and her needs.
There is no real cut-and-dried answers to your questions, so let me just tell you about my situation in the terms of your questions.
A. Can I ever really trust her or believe what she is saying to me?
I still don't trust my ex and I never will. Just in the last week she has sent another letter to me this time apologising for her antics during and after the relationship. She stated in the letter that she has no "ulterior motive" in her apology ... but I know that her divorce will be finalised pretty much any day now ... I feel that she is trying to soften me up for more attempts from her to get back with me.
B. Is she definately going to cheat on me?
I don't know if my ex ever cheated on me, but I am suspicious about one weekend. However I do believe that if I continued in the relationship she eventually would have. She does have a history of cheating on her (soon to be) ex-husband ... including her affair with me.
C. Is it possible for her to ever really care about me or is this going to be simply a case of "she needs me to take care of her"?
I think with mine she sort of did care about me ... but I feel it was more in the context of "if I care for him, what's in it for me? .... what will I get out of it?"
As far as making it bareable goes, a relationship with an HPD sufferer will be difficult. Read my post
http://www.psychforums.com/viewtopic.php?t=11676
about half way down the post is my view on how it would be to deal with an HPD long term.
I stress MY VIEW ONLY. I am not an expert.
Good luck.