There will be a Luna Engine Plugin in the future for MV but the current one is only compatible with Ace
LUNA for Ace is in RGSS3 - it can't work in javascript/MV. But there have been indications that Archeia works on a LUNA for MV, but that would be a different product.
You might like to double check, but my memory is that the pre-order page says that one of the pre-order bonuses that it comes with is Luna Lite, so you would have something to start with. ninja'd by the high-velocity squirrel.
My question is now, will we have to buy it for MV if we've already bought it for Ace? If so, will there be a discount? :3
Yes, it'll be a separate product so it will be a separate purchase, and I doubt there will be a discount but we won't know until it comes out or until Archeia says otherwise