To me, this screams of an emotional issue, not a pysical one. I'd bet that Lex is an intesnly emotional, sensitive child.
I'm sure her social setbacks are not about intelligence, but about feelings. She watches the world around her and can't imagine anyone feeling the way she does, so she doesn't think she can relate to others. It sounds like her biological mother is unpredictable, so Lex probably has issues with trust and depending on others. Her "mother hen" play tells me she has good social instincts and also, I think she's acting out what she wants from adults around her. Her clingyness tells me that she wants attention and praise, more than an average child would.
I think the bedwetting is juat a symptom of what's going on inside her. She seems to be crying out for help. It's very normal for kids her age to bedwet, tantrum or act out strangely when they need help but they don't know what or why. I don't think she can help it. I'm sure she's terribly embarrased about it.
I think she needs to see some sort of behavioral therapist or psycologist. If you were to go that route, you might start by talking to her school councelor or pediatrician. Most of all, though, she's lucky to have you in her life. You want to help her and you love her, I think that's what she needs the most.