Yep, she sounds like my difficult child. Very picky eater, but will eat when hungry enough. He also likes for his ice cream to be melted. He's had a "grown-up" flat stomach since his second birthady. He's still skinny.
She sounds very low tone (natural muscle strength) and this causes her arches to be flat and have trouble with gross motor skills because they require strength. My difficult child will often just wait for someone else to do stuff for him because it's just too much trouble to do it himself. Sounds like that could be part of the stairclimbing behavior. Although they CAN do it, it's worth it to wait for someone else to help.
He would also sit on the periphery and just watch to see what's involved in an activity before he joins in. He's pretty much grown out of that, but does get nervous before new things, such as the first soccer practice of the year, etc.
About the W-sitting, please try to stop her from doing that as much as possible, especially when sitting for a long time. What happens is that her bones are still so pliable and soft, and her thigh bones actually grow twisted. This makes her kneecaps turn in, forcing her to turn her shin bone out, which causes her arches to be even flatter. Try it with your legs. Stand up, turn your knees in, then try to point your toes forward. Your ankles roll in. My sister showed me. My son, thankfully, wasn't prone to W-sitting, but his thigh bones still turn in, causing undo stress on all of his joints and they can't handle it. It's important that all the bones line up as close to normal as possible, when you don't have the normal joint strength. Don't mean to scare you, but it's traumatic watching my son with his knee locked up, screaming in pain. He, of course, is the only child I know of that has joints THAT loose.