Yes, because she is a cool character and hasn't appeared since 2008, except for being a Smash Spirit. It would also be a good opportunity for Wario and Syrup to interact with each other verbally, because Wario talks more in the Ware games.
Yes, but only as an antagonist, not as an employee. (Which Wario would never allow of course, but still, I hope that no one at Nintendo gets this idea in their head. WarioWare has more than enough characters anyway.)
I can imagine Wario getting kidnapped or something by Captain Syrup, and Mona going to rescue him, with the rest of the crew soon following. Or something like that.
Yes that could be cool if they did it right. Unfortunately how Captain Syrups Character is unless she's stealing or dupping wario to steal for her to my understanding she has no interest. I really couldn't see very many scenarios where Captain Syrup would enter the microgame industry. I like the kidnapping idea but doubt that the Black Sugar Gang could successfully kidnap our champ. Maybe she sees that WarioWare networth/stocks are worth a lot so she kidnaps some less powerful characters and ransoms them for stocks but without Wario the whole crew falls apart .