Headscratchers: Professor Layton and the Eternal Diva
One thing I really didn't understand, and was never explained to my satisfaction, is why Amelia was chosen. Melina's father notes that he 'searched far and wide' for a 'special young woman' to receive a free ticket to the opera. Of course, the real purpose was to find a new house for Melina's memories and personality, I get that part - but what was so special about Amelia? She was supposed to be a gifted chess player. That didn't exactly guarantee that she'd have the vocal range necessary for Melina to sing the Song of the Sea. So why did they pick her?
I thought they picked her because she was capable of housing them, which was what was most important to Oswald, anyway. Not sure exactly why she was compatible, just that she was.
Presumably her compatibility was somehow linked to her intelligence level. She was, after all, one of the few to solve nearly every puzzle first. That would also explain why Oswald briefly tries to use Luke, who also did very well on the puzzles, as a fallback for his plan.
In regards to the Song of the Sea part, the vocal range didn't seem to be the issue. The problem was that Melina was the last person to know what the song was, so when she died, all knowledge of how to sing it was gone. Putting Melina's conscious in someone else's body would ensure that there'd be someone alive who could say what the song was. And if Amelia's body didn't have the physical ability to sing it, they could have just ordered her to write it down and found someone else to do so.