The HU does sound like it’s working in reducing her platelets, but it just seems this is pretty rough on your mom. I’m sure this is frustrating for her doctors as well as your mom and ultimately, you because you’re so worried for her. I hope they might consider one of the other potential medications to see if they’re beneficial with possibly fewer side effects.
It’s not easy when you live so far away from your mom. Does she live by herself or in an adult/assisted living community?

It might help to know which driver mutation your mom has and what her platelet counts are now. If I were 81 and had the CALR mutation with platelets below 1,000, I might decide to forego treatment and forget about the ET altogether. In that situation, I would not be likely to throw a clot (if I had no prior clots) or to get acquired Von Willebrand's from the high platelets.

Dad had ET and stopped treatment when he went on hospice for end stage emphysema at age 81. He lived another year without problems from stopping ET treatment.

I don't doubt your mom is having a reaction to the HU, but I would also wonder if your mom is having some severe emotional responses to her diagnosis and feels alone. My mom and her sister visited the ER frequently in their 80s before being diagnosed with mild dementia. They were overwhelmed by their health problems (heart disease and COPD), and the ER was the only place they were getting reassurance.

I know it is very hard to have an ailing parent far away. You might contact senior services in your mom's locale to see if she can get a visiting nurse or just a visitor who looks in every few days.

Please don't take any of this as advice, but it might help you open up lines of communication with docs, mom, and potential helpers.

So sorry your mom is experiencing such negative side effects from hydrea. Question: What is your mom’s platelet count? I ask because I was diagnosed with E.T. in 2021 with platelet count in mid 500s. I chose not to take Hydrea and get blood tests every 3 months. I also take a baby aspirin every evening. My platelets have stayed in the mid 500s and I feel fine. I am 82. I mention this since the doctor said the alternative for your mom is to do nothing. It sounds as though your mom is emotionally and physically wrought with the side effects of Hydrea. Depending on her platelet count, perhaps doing nothing may be an alternate plan. I do not want to sway you/her in any way. Just want you to know this is an option you may want to discuss with her doctor. Wishing your mom the very best outcome.