The No Cry Sleep Solution for Toddlers is very good. I would try reading that and see what is suggested.
This sounds like a child who has built up a lot of anxiety about the idea of either going to sleep or being left. That's why she can't fall asleep and why she reacts so strongly to you trying to reduce any of her comforts.
I would personally not want to do sleep training in terms of rapid return, sssh pat etc as I think this would potentially make the anxiety worse. I'd try and go back to how things were before when it only took 20 minutes, even though in the short term that will mean it takes even longer.
How is her language? Can she talk to you about how she feels at night? I would guess not (very much).
Restless legs is very rare. I don't think it's likely to be that. But as it's related to vitamin deficiency, a gummy multivitamin for children isn't likely to do any harm and might help. If you can get one with Vitamin D (if she isn't already taking that) I'd recommend it.
Could you move her to a single bed rather than a cot? Maybe a mattress or futon if you are worried about her falling out, or just use a bed guard. Then start her off in her own bed. You could still lie with her if you need to. I used to tell DS at that age if he started chatting or moving around "Ssssh, I want to go to sleep, I can't sleep if you do that."
When he was a bit older it was a clearer "You need to lie still and quiet." I explained to him before I tucked him in that if he started to make noise or move around, I would count to 3 and if I got to 3 then I would leave. I found that he simply didn't know how to relax into sleep so I had to explain it to him. FWIW other children may need to move or hum to themselves etc to self soothe - pick things that work for her.
If you do choose to move to a new bed you could use it as the start of a new routine. Instead of lying down you could promise to stay close but do something like a sleepy time massage instead. You could try a "magic sleepy spray" (few drops of lavender essential oil in water in a spray bottle) It might also help to have some kind of audio cue for sleep as well, either some calming music (try new age or meditation music on youtube/spotify) at a very low volume, or books on CD, again at such a low volume that even moving in bed rustling the covers would obscure what is being said. That encourages her to lie very still to listen to the story. Over time once she comes to associate this music/story with the calming presence of you and the massage/cuddle and as she will be older you might agree to shorten the massage and sit with her for a while instead, eventually getting to tucking her in and leaving.