What I would be concerned about is the duck heating all the way through again without drying it out... especially in the microwave, which is notorious for uneven heating.
The rice probably won't be a problem, nor the spinach.
The sauce - I'm assuming a classic green peppercorn sauce that is cream-based. Definitely don't microwave that, the sauce will break. And reheating can be difficult because it can still break when reheated on the stovetop... getting it to emulsify again can be problematic. Be careful about that. Bring it back up to temp low and slow and whisk like crazy.
What I would do (personally), is to do all the sides ahead of time and have cold hors d'oeuvres with the cocktails if you are having them (I'm assuming you are not having a sit-down appetizer, which would probably overly-complicate). I would have the ducks roasting and time them so that mid-cocktail hour, they come out to rest. You can start reheating the other foods when you pull the ducks to rest and complete your sauce then.
Good luck! (I agree that for that many, depending on the size, you'll probably want at least 3 ducks.)
Life is too short to eat processed, artificially-colored, chemically-preserved, genetically-modified food. Or maybe that IS why life's too short.