It's been implied that Time Lords (other than the Doctor) can control their regenerations. The Master intentionally made himself much younger, and then there's that episode back in the 70's where Romana cycled through a handful of forms during her regeneration until she found one she liked.
I think while Melody is still in the middle of regenerating, she has some control over her looks. That would explain everything except why she regenerated into a toddler in the 60's but was still a child in the 90's.
I'm gonna skip over the space suit theories, since there are about 12,000 different ones floating around these threads.
I think Mels knew she wanted to kill the Doctor, but she was also intrigued by him from Amy's stories. He was the target, but her mother also made him sound so wonderful. So when she met him, she wanted to see him in action before she killed him. I think that's why she waited until she regenerated into Alex Kingston before trying to shoot him.