I too welcome you here to the place no one want to be, but please take heart, it is the best place to connect with those who have been there and can support you.
We too had issues with standing all the time and it is hard to extinguish. We kept feeding and like tina72 says, we distracted and made her sit without her knowing. We took her on a canoe ride for three hours. We made her sit for one hour every evening and then after a few days added a half hour. She used to yell at me so her father would sit with her for the time watching Netflix videos. At first she would time every last second and then once she got used to it ie distress tolerance and got over the first few min she did get better. She would try to get up and walk about "I have to use the toilet etc.." and then we would just tell her she had to sit and if she got up we added that time on top.
We traveled too and she had to sit on the plane of course. I does get better
You say "One of the things I struggle with the most is knowing how firm to be. I try to show empathy and be firm but caring, but I think I get so ground down with it all I am not always firm enough. It’s very hard, and it’s lovely to be able to write about it in a place where other people understand, as I know no one who has been through this themselves." I had to change how I saw parenting. I had to be the tough lady when it came to food. NO NEGOTIATION AT ALL. It is counterintuitive, I know, but it has to happen. Until I got my mind around it, it was really hard.
Having a plan if she does not eat, drink, if she throws things, etc.. was helpful and took time. (get plastic plates in case of flying objects)
Use incentives, like phone, friends, shopping, makeup whatever she likes to get her to eat. "You may have your phone when you eat, you may call a friend over when you have eaten". Some do well with friends to eat. My D ate like she was normal when others were around. If the friend had pizza she would too. You have to learn from trial and error ( a lot of errors really helps us learn) what works. Torie said in one post recently and it is so good, that you have to learn how to make eating the only option and that eating really is the preferred option than not eating. So if she want so see her friends, it is easier to eat than not to.
Keep asking questions.
Food+more food+ time + love +good professional help+ ATDT=healing---> recovery (---> life without ED)