When they get like this, and are not responsive, or nearly not responsive, I don't force anything. Too much stress, and too much chance they will aspirate. Just hold her, and love her. I'm so sorry.
Please don't be hard on yourself. It's entirely possible that you had nothing to do with this and it's just the natural progression of whatever is going on with her. I have worked really hard on trying to save some of my birds only to find out at necropsy that it was really out of my hands...
It's ok, I know it's hard to lose them. And to feel helpless. :hugs
The issue with flushes is that they are very dehydrating. If she's drinking a lot, then that may not be what you want to do. It basically gives them diarrhea, and you have to make sure you replace the fluids adequately...
I don't think it's likely that you overdosed the vitamins, but if she already had kidney or liver issues, then possibly it could exacerbate that. Many of the supplements for poultry are very helpful in the short term, but not really intended (or good for) longer term use. It's very hard to say...
I personally would not do the vinegar. Clean, fresh water is really the best thing for her. Her digestion is probably slowing, so that may be causing most of her issues, as well as not absorbing nutrients as well. If you want to give probiotics I would use an all species one, like probios...
They don't get acid reflux, they can bring stuff back up if the crop is not emptying or if digestion is slowing due to another issue farther down the digestive tract. The jerking may be crop adjusting, that is common if the crop is very full, they are trying to get it to move and empty...
Age may be catching up with her. No matter how spoiled she is, you really must stop the treats, they dilute the necessary nutrition she needs to be healthy. Vitamin deficiencies are very common and can cause very weird symptoms. An older bird may not be absorbing nutrients as well, and that...