You don't have to fly both ways with the same carrier (and you can also make a stopover somewhere en-route if you like). So why not try two Flying Blue airlines?

Personally I'd avoid Delta and, if they were the only flights available, I'd change my dates. I haven't flown KLM Business. I thought CO not bad, but if you really did get offered the VS codeshare, then I'd definitely give that a try one way via LHR (preferably on the return journey to take advantage of the flat beds and also to avoid the massive long-haul passenger service charge if that is payable by connecting passengers departing Heathrow).

Are you allergic to Alitalia? I thought their Business Class good on a Tokyo-Milan flight, but I don't think the seats would be very good for an overnight flight so I think I'd avoid them coming back. But why not AZ out and AF back?