In my head this totally probes my headcannon. If River is half-Time Lord and had the ability to regenerate, wouldn’t Donna have the ability to regenerate as well? Then it finally hit me.
Amy IS Donna
Donna regenerated into a seven year old ginger girl (this is also my theory of why the Doctor is still not ginger… Donna/Amy stole it). Donna’s regeneration power protected her brain by resetting it, so her brain wouldn’t explode if she heard of the Doctor. This explains how Amy didn’t know about the Daleks or anything else. However, protecting her mind used up all her regeneration power. This is also the reason Amy’s mind is so powerful. For example, she was able to imagine the Doctor back to life.
Now if Donna’s brain is wiped, how did she get to be Amy? Well when she was found with absolutely no idea of who she was and a Scottish accent, the authorities told her her name was Amelia Pond and that her parents died, and that she now lived with her aunt, who in real life was he adoptive mother. This and her mind is why her “parents” look so different from her. She imagined them to look that way, based off what she thought her parents should look like and her “aunt.”