Hi Boogie74 and welcome back

It's good that you're able to see that you played a part (not all, not none) in the conflicts that happened. Nothing will change until we make changes on our end -- knowing that you contributed (again, not 100%, not 0%) is the start of being able to change the dynamic.
Today i suffered a small setback that i could have helped prevent- I continued and enabled an escalated argument which I am absolutely confident that I was correct about- yet winning the argument would be of no benefit to either of us. UPS delivered a package to the wrong house and she showed me a GPS map that detailed exactly our house on the road. She insisted that the GPS showed next door and I (feeling starved of the opportunity to be right about SOMETHING for once) tried to explain how the map was correct. She insisted it was wrong and that I was wrong and down the rabbit hole we went.
We all want to be right -- it feels so good -- and we want support from our loved ones in being right. It really hurts to have a fundamental "right vs wrong" conflict with a partner. I'm sure you wish you could just have a moment where J says "you know what, yeah, you were right about that".
It sounds like you
both have high needs to be right. Does that ring true?
If you were to go back in time to that situation, where would you decide to try something different? What do you think you would try instead?
I eventually changed the subject- “want something to eat? I’ll reheat some Chinese food”. This is where the wheels came off. J is consistent in her false assumption that if I loved her, i would just “know exactly” the methods she uses to reheat food (in a ceramic dish versus the plastic one), whether I reheat her sauces in the microwave with the food or afterwards, etc.
If BPD is in the mix, then consistency in false assumptions is part of the package, not an aberration. The issue is if you are able to
radically accept that your loved one will continue to have false assumptions. I'm not saying it's "acceptable" that she does -- I'm saying that that is who she is.
She saw me put the plastic container in first and became upset saying I needed to heat the sauce (in a separate container) WITH the food. I asked for clarification- explaining, “there’s a reason i started to do it this way- but I’ll do what you want” so I asked “Do you want it in a separate container or on top of the food?”
I got the words “Do you want…” out before she interrupted and told me to do it the way everyone does it and to stop asking stupid questions. I complied as best as i could and poured sauce onto a dish of food and asked again, “Is this how you wanted it?” (wanting to know if it was enough sauce or if i should mix it).
She went absolutely berserk- “You ruined it! I won’t eat it! You don’t know anything about me if you can’t see how I do that! Have you ever seen me do that??? NO!”
She then went into a crusade of insults, projections of me being a narcissist- because you can tell a narcissist because they deny being one and they think about themselves and say things in response to others talking to or about them… (The usual routine).
How long did you stay in the room to hear the insults?
My question is this: I assume some others here deal with situations where simple information is not shared with them by the pwBPD and small activities and plans are “destroyed” because you can’t read minds and you just don’t know what they want. HOW do you handle it without causing a WWIII blow out about “how should I make your eggs?”
These are features of choosing to love someone with a mental health challenge like BPD. It must be difficult and frustrating to run into her limitations in areas that seem simple and small.
I'm not sure that there's a direct answer to handling the exact situation, but what I am thinking is that there's a bigger picture balance to aim for.
One part of the balance is going to be emotional validation coupled with
genuine curiosity/empathy/interest, to create a more trusting environment. If she can feel like she can trust you to care about what she really thinks and be gentle about it, that may create a sense of security where she feels safer being wrong about stuff. If she feels like "you don't really care what she thinks or why", then it'll be a lot harder for her to let go of rigidly holding on to "she must be right".
The GPS thing comes to mind. OK, she's "obviously wrong", but leading with that will destroy a trusting environment. There's no rush to jump to "let me explain to you why that's not the house" (nobody is bleeding to death, you aren't an EMT team rushing to an accident). Why not take a moment to learn from her -- genuinely -- more about why she thinks that's the house?
Her: shows you the GPS map
You: "so you're thinking it's X house? Is that because it looks like ABC?"
Her: "Of course! see how it shows it exactly?"
You: "well, I'm willing to give it a try -- want me to go with you to get the package, or want to go yourself?"
If you don't have time to go, it's fine to use a personal boundary -- kindly -- to share that info:
Her: shows you the GPS map
You: "so you're thinking it's X house? Is that because it looks like ABC?"
Her: "Of course! see how it shows it exactly?"
You: "I can see how that makes sense to you. I can't go get the package today, but I will have time tomorrow starting at 11am if you can't grab it today."
...
The food situation strikes me as one where the balance might be a little less tilted towards validation and a little more towards your boundaries.
If she was already "on the runway" revving her engines to launch into insults, then you get to decide if you'll allow yourself to stay in the room and listen.
You don't have to explain to her what you're doing or why -- that can tend towards
JADE-ing.
You are allowed to take a break to preserve you and the relationship from negativity.
Her: “You ruined it! I won’t eat it! You don’t know anything about me if you can’t see how I do that! Have you ever seen me do that??? NO!”
You: "I need to go to the bathroom, I'll be back in bit"
or
You: "My head is starting to hurt, I'm going to the drug store for aspirin. Should be back in an hour"
...
What sounds doable?