It certainly does look like a pull up to me. That is what my 5 grammy kids have done while wearing a night time pull up, they get up and take it off, then put pj bottoms back on. I could see CY doing this when she woke up.
That's what I thought. I spent a month with my daughter and her family not too long ago and I had to help with bedtime for children aged 2.5 and 4. It was standard to put a pull up on the 2.5 year old. In the morning, he took it off. Children also tend to leave the pullups in the wrong place. I would interpret that (if it is a diaper) as the child taking off her own diaper in her bedroom in the morning ... before she was in her mother's bedroom. If that is a diaper, why would investigators state that someone cleaned and changed the child? If it is a diaper, how do they explain that it is on the floor?