carpe How do you deal with your partner? Is he able to be open and honest about his feelings or does he respond best to emotional blackmail and manipulation?
Deal? Ha! Oh that it were something you dealt with rather than lurched from low point to end of tether-itus with. 
I guess he does respond well to emotional blackmail. Ought to, he had a full 45 years of training right from when he was a few days old. I stopped any kind of "deal with her this way or I'll....." pretty early on. I didn't want to pile more of what she did on him. I wanted him to know something honest, rational, fair and...actual love in action rather than just an empty word used as a chain and padlock. He knew when I was feeling squashed, highly insulted (I am/was just like the SS, Gestapo in the shagtastic suspenders of an English whore doncha know) and frankly at the end of my rope. But we sort of just muddled through, with rows, tears and screaming mathces as part of the landscape when things got too much. But then, he also knew that I felt for him. Cos I knew it must be so hard loving somebody who only knows how to manipulate in return.
He is still grieving right now. She's only been dead since just after xmas. But pretty much I think the reason why things have been so good (even with grief factoered in) is becuase we aren't repeating the cycle thanks to bad habits cultivated during her lifetime. Perfect we are not, but the foundation of our interactions are based on real feelings rather than "cunning plans" being put into actions.
I think pretty much if you want this you either accept you will come trailing at the end of priorities (not becuase he loves you less, but becuase he fears her love more and has never known anything different.) or you do what all the rest of us do. Spend several years fighting it oly to end up in a knackered, dispirted, almost broken heap and face up to being the "model of good love" while wiping all the crap thrown at you off your face at regular inverals. and fantasise about dancing on her grave
Which incidentally you don't get to do. Well I didn't. Problem with people this ill is that when they get old and frail almost no OAP home will touch them with a disinfected bardgepole.
So ...family it is.
As incensed as she got me, ...you can't spend that long with somebody depending on you, even if their gratitude level is sub zero...and not get some kind of attachement.
One of the biggest irritations I have is that I ended up feeling so sorry for her. For the good, healthy life she didn't have becuase of the way she wad. And bits of my heart got a bit squished when she died, cos that meant all last hopes of a tiny slice of "better" were gone.
You want nice, clear, simple answers. Which is hugely understandable.
The problem is, there aren't any. If she is ill or this is chosen, but deeply ingrained, behavoir, it's pretty much over bar the shouting at this point. This is how she will be, your love interest will likely keep repeating the habits of an entire lifetime, you will probably keep searching for solutions that don't exisit.
Untill you give in, accept, and work with what you've got rather than what you wished you had.
Nothing going to help except time and experience and .... a mule like stubboness that she isn't going to ruin your relationship with her son.
And maybe running screaming for the hills. That might help. I wouldn't blame you if you did. Make sure he is worth it love. Becuase it is quite possibly going to be very expensive emotionally, loving this man longterm.