Elephant- i think you were very assertive to say what you wanted to do over lunch.
Maybe you need to be more open but in a light way- eg above example- could you have added "Don't be offended- still love you- just want some time with DD...byeeeee"- that kind of thing?
I think it is vital that you don't allow your behaviour to be controlled by your parents. They obviously have issues, but they needn't become yours as well.
I think you have found the answer it would need to happen in a very small way at first.
On the other hand, can't you actually confront their behaviour- can't you say things like "I can tell you aren't happy about X,Y,Z- let's have a chat about it- say what you are thinking".
I can't see that would equate to being bossy- more acknowledging their feelings.
They sound very non-confrontational. I'm afraid it is up to you to make a stand and bring your feelings about it into the open. They might be offended- but,no pain, no gain - they will re-adjust, but I can see they will be upset to begin with.
You see, you have been colluding with them all these years and reinforcing their behaviour. They won't see it as that,but from the outside that is very clear.
If you hear them talking about issues behind your back ( do they WANT you to overhear perhaps???) then why don't you tell them that you have heard and say you want to talk it through- that's why I mean you collude- you accept how they behave and don't challenge it.
As for your other question- yes, I did find the rows upsetting, but they clear the air. I can't imagine anything worse than bottling it all up and not communicating. It's a case of having "words" then having a hug afterwards- and that's how I'd like to think it is in my house now with my own DCs.