I'm frustrated and getting nowhere ...what can I do about Mom's unintentional weight loss and negative, passive/aggressive behavior?

It may be with that amount of weight loss and mental decline that your mom may be further along the stages than you realize. A memory care unit would be most helpful, but if you need to keep her at home, then I'd ask her doc to medicate her as they do in Memory Care. There are so many on anti depressants for the various brain dysfunctions. Then there are anti anxiety meds that help SO MUCH when the sundowning is going on. Mild drug help when there is evening restlessness, but not actual sleeping pills which seem to be the blame for broken bones in the elderly. There are so many ways to make our elders more comfortable with the brains they have. Please remember that the warnings on the drugs are for people in their right mind and our elders don't have that anymore - their brains are broken and we need to help them out, no matter the potential side effects. For their quality of life, if a drug gives them more happy days and maybe makes their life a small bit shorter, I am sure that mthr would prefer that to being miserable and scared for that extra length of life.

My mother started showing signs in the late 1990s and was diagnosed in 2002. She was always very energetic, active, and independent. So as frustrating as is is for my sisters and me to have to care for her, it has to be doubly frustrating for her to need care. At stage 4, she should not be left alone, even for an hour. In the time it took me to put a load of laundry in the washer, my mom had done things like turned on all the burners if the gas stove. Or tried to use a large kitchen knife to open a can of soup.
Try to remember she is not doing things on purpose. Her brain is no longer functioning like an adult brain. I compare my mom to a child. Right now, your mom is probably like a kindergartner. Possibly younger. My mom is like a 12 month old on good days, an infant on bad days. You have the crazy task of treating them like an adult and a baby at the same time.
You need to have more help. See if family members will take over on a regular basis so you can run errands, get groceries, whatever. In my family, we have regular 12 hour shifts so we can make sure mom is never left alone.
As for UTIs, it's a constant battle. Give you mom whatever she likes. If not plain water, then ice water, or juice, or iced tea, or even just add some flavor drops. Figure out what she likes. You can't expect her to understand that she needs to drink a certain anount. You just need to make sure she always has water in front of her. We used to use a fancy thermal cup with a straw but mom can't always remember how to use a straw now, so we just use small cups and refill constantly.
Keep plenty of high calorie snacks on hand. When mom's tremors were bad, she lost 60 pounds. We now give her all the ice cream she wants. Plus cookies, nuts, fruit, yogurt, chips, whatever she likes. Her weight is pretty stable at 110 pounds. Still too thin for someone 5'10", but she's not losing any more.
And last but far from least, schedule time for yourself. Get a family member, friend, sitter, and take a couple of hours every week for you to just be you. You can't take care of mom if you aren't taking care of yourself.
Two final hints, I separate mom from her disease. When mom is difficult, it isn't her, it's that mean old Mr Parkinson again. And I have a "bucket list" of things I will do when Mom kicks the bucket. It's not morbid, it's a reminder that you won't be doing this forever.

It sounds like she has Parkinson Plus symptoms. A lot of these behaviors are due to her dementia, not passive/aggressive behavior. She likely needs an antidepressant. As the brain shrinks during dementia, it is harder for the synapses in the brain to do their work and an antidepressant will assist. If her parkinson's specialist isn't helping, look for a good Dementia/Alzheimer's Care Center. Even better is to look up Lewy Body Disease Centers of Excellence and see if there is one near you. LBD is on the Parkinson's spectrum.

Mom may need meds for the afternoon problems. Try cranberry tablets for the UTIs. Have Moms Thyroid checked. It gives off hormones that when not working right can cause all kinds of problems. A hyper one will make a person lose weight.

Fruit juice or Kool Aid type drinks, instead of water, could help with the two issues of not enough fluid intake plus give additional calories from sugar. If it tastes sweet, would your mom drink more liquid? Even if she doesn't drink more, it could help keep her weight up.

I had the reverse with my mom and Dad but I couldn’t afford memory care unit but did get palliative care thru Medicare. Be glad that you have help I had to beg and plead just so I could go Grocery shopping and my dad would stand over my stepdaughter and make her clean. Not a bad thing for her since she is terminally lazy but you can imagine how willing she was. My own daughters would breeze in and breeze out. Once the sundowners kicked in it wasn’t long. If she’s losing weight like that, not to be insensitive but it won’t be long. And remember that those UTIs make them nuts in the head. My mom had 9 in the 6 months before she died and once they diagnosed it as part of her benign MS it was 4 more weeks. I’m sorry I sound so terrible but it’s over a year that they’ve been gone and I didn’t plan for myself after.

My dad, 94, is suffering from Alzheimer’s and congestive heart failure. He is in an assisted living home. Lost about 30 lbs in 6 months. He has good days and bad. Sundowners is a daily thing. He’s “with it” for a few hours in the morning but as the day progresses, he becomes mean, more forgetful, and very negative. We now have hospice with him for several hours a day. It’s been a God-send. He still hates it, but it gives me peace of mind

Sounds like your mom is beyond being able to deal with the pitcher of water - you may need to fill her glass and set it in front of her. My mother with short term memory problems doesn't act without prompting much anymore concerning food. She will skip eating lunch even though there's plenty of prepared food in the fridge that she just needs to take out or reheat in the microwave. If I ask her to fix herself some cottage cheese and tomatoes because I'm stuck on a conference call, she will fix it and eat it but I don't prompt her it just doesn't happen. I've started preparing/storing clear serving sized containers for the fridge. The counters have fruit and cake slices. There's a "dump cake" made from pie filling and a yellow cake mix that she likes so I make one just about every week. She eats from the counter better than the fridge. When I home, I open the fridge and get a container of watermelon or an ice cream sandwich out for an afternoon snack - she almost never declines the food. Usually she drinks enough without prompting but I still keep an eye on fluid consumption. Coffee is the one thing she still cannot live without - fresh pot every morning and she reheats cups in the microwave after the warmer times out.

Besides drink alternatives, maybe soups and fruits high in water? You might google "healthy foods to gain weight". Have had temporary situations with both my parents when they had unwanted weight loss. A couple of times we resorted to buying Benecalorie, a supplement than can be mixed into foods to add calories. I know the last time I went through this with my mom, I thought she was going to die because she was so weak she would not have been able to fight off any infections. She was also very negative and unreasonable. She was much better once she gained a few pounds and started feeling better. She's still negative and unreasonable, but more at a level I'm used to.

Sorry...didn't read all the other answers...am very much in your boat!

I was resistant for a long time to "serve" Mom in her bedrm as I wanted her to get up & walk around. Her whole life being of Finnish descent she and her friends would enjoy "afternoon tea or coffee." One day out of the blue I asked her if she wanted her afternoon tea to which she excitedly replied " O yes please...that would be nice!' Purely by accident I discovered how to get her to consume 2 cups of tea "on her terms bedside." Took me 2 yrs to figure this out!

I sense tho that the crisis is with you right now. Get those family & friends to help out more NOW. You are SO lucky to have them.....USE them ..they are your lifeline. I suspect you have sugar coated things to them as I did for a long time. It's a "protective mechanism" of your Mom.... so natural to want to do it all..of course you can't & great that you have come to that realization as your Moms care is escalating. She is so lucky to have you....you are her world...

Let helpful family and friends (or just start with who ever is closest to you) know how you are @ the end of your rope & would so much appreciate an afternoon off. You might start crying as you feel the immediate release ...just go with it.

You must get away for an entire afternoon this week. I know you are mentally & physically exhausted so don't plan anything taxing for your "time out." Even if it's just going to the park..laying on the grass...watching the clouds go bye like you did as a kid yourself...and letting it all go. I did just that a few days ago and it was so therapeutic....

Good luck my dear....you are doing a great job.....xxxooo