Please help me understand the end to Eternal Sonata. (spoilers)

[ROBOTECHHEAVEN]

the ending was about death and how ppl view it. and about polka , she was a character in fredericks, whom he saw grow up and fall in love, also seeing how she did things. he couldnt let her die, also i think in his mind polka is fredericks version of his little sister who died of a disease, which is similar to the one polka has. so he decided to allow polka to keep on living so she could experience what his sister couldnt do in her little time she lived. and as for who is singing opera after frederick dies, that i dont know.