The fare you are quoting above is an AF MHPJ09M2 fare. DL has an equivalent fare filing, but the Flight Application rules are different. The AF fare permits WS flights, while the DL fare equivalent does not.
You cannot force routings on a single fare component with multi-city search on delta.com like you can on ITA Matrix. Delta has configured their multi-city search to force every break to be on a separate fare component. This means layovers must be at least 4 hours on domestic flights and 24 hours on international for each break. The only way to get it to work with multi-city is to have an overnight layover at YYZ or LGA (to create the fare break) and book the award as two separate fares. In the case below, FSL295 for BOS-YYZ award and a CMSH1200 partner award for YYZ-ATH (29.5K + 120K = 149.5K miles). The award fare construction is broken down at the "Fare Rules" link at the bottom of the flight listings.
The cheapest solution is to simply to book two separate award tickets and don't bother with multi-city search. One for BOS-YYZ on WS non-stop in Economy (WS does not have business on this route) at 12.5K + a separate award in business on YYZ-ATH for 120K for a total of 132.5K. You also don't have to have an overnight layover if you book as two tickets.
