Two options:

Book business outbound, economy inbound (with E+ seating).

Theory:

If you do propose, she'll be so giddy she won't care about the flight back.

Alternatively, book economy outbound (with E+), and sleep an extra day before you propose. Then book a business flight back.

Theory:
That might be a nicer way to do it, as once she's engaged, she will feel like a princess. She'll want to show that rock to every female FA working on that flight home.