Why are you so worried about sounding like a complete bitch? Have they ever worried about how the way they treat you makes them look?
Stopping people from taking advantage of you does not make you a bitch. Protecting your own DC's interests does not make you a bitch.
Stealing from your daughter and your grandchildren, consistently expecting your daughter to put you first while yourself always putting her last, however, makes you pretty damn low IMO.
I think you need to take off the rose tinted glasses that you're looking at your parents through. Just because they think they're good, loving parents and grandparents doesn't mean that they are. I'll bet even Rosemary West thought she was a great mum.
You're an adult now. You get to decide if they are/were good parents or not. I suspect the only reason you're not yet quite willing to rip off the plaster and see what's really there is that you know it will hurt. It does hurt to see that your parents are very inadequate people who don't really love you, and people will go to great lengths to avoid facing up to that. You yourself have sacrificed thousands of pounds and a great deal of your own comfort in an attempt to stave off the truth: what else will you sacrifice?
You absolutely can kick up a fuss about all this, it's just that they've got you so well trained, you think you're the one being heartless and unreasonable, when all the time it's them. Emotionally abusive parents do this to us, all of them, in their different ways.
Your parents are perfectly capable of being cold, ruthless predators when it suits them. Acknowledging this as a fact will not be easy for you, but it could save you a world of pain in the long run.
The bottom line is that your parents will always put their own welfare and best interests ahead of yours/your DC's whenever there's a clash. It doesn't matter how nice they can be in other ways, if they consistently prioritise themselves above you, when the chips are down, that's a huge indicator that their "love" is a mirage.
I know this might sound harsh. I don't mean to sound like I'm being critical of you, not at all. You are clearly the good guy in this scenario. But I think that instead of focusing on this one visit, you really need to reassess your whole relationship with them, and give some serious thought as to how you want it to go on from here. As regards the visit itself, it's pretty obvious I'd say don't go, I think!