All this is asking a 5 year old to behave in ways that she has never had in her life. And she hasn't got any structure either. As wonderful as you are for being her carer, her sad reality still is that she has a temporary home with someone she fears bonding with and her Mum has left her. Given these circumstances, is then trying to make her behave in a disciplined and structured way, fair on her?

The behaviours you have described are hardly excessively naughty either imo, and I believe they're maybe linked to testing to see if you CARE about her, and not wanting to hurt you.
Leaving the house = would you care if she was gone?
Not eating = do you care that she's not eating?
I think she needs emotion and love and to feel she matters, and not punished. That sends a message that it is bad to look for that reassurance or try to express herself (dysfunctionally, true, but the only ways she knows how )

I think it's the same with the sleep issue maybe. If you spent two years being abused by someone, would you want to be put somewhere with no distraction or noise, alone with your thoughts and unable to sleep? no, me neither. And if I couldn't communicate that, maybe I would get distressed and scream too.

I think you are amazing for caring for this little girl and very warm hearted. You are also extremely brave to ask for opinions and ideas on how to do your best for her. I hope you find the way that works.

Some ideas about dealing with traumatic bedtimes ..

Give her a night light that shines pictures on the walls
An ipod with calming music on and stories that come with the books
Buy her a magic torch (tell her it's magic)
A dream-catcher and tell her it catches happy dreams and keeps the bad ones away
Buy her (or make together) a worry dolly, that she tells her worries to and puts under her pillow and while she's asleep the dolly takes her worries away.
Does she have a night time teddy? Get one from the charity shop and tell her that someone had given the teddy away and he looked sad, so you brought it home so the teddy had someone to love again and a nice home