Although many people will tell you that there's no reason to do a platinum challenge if you don't plan on flying enough to keep the status, I think that your idea of splitting years with your wife is a good one. If you always fly together, there is no reason to both have status. You can check-in/board/upgrade together.

Doing the challenge before June is a good idea if you plan on flying a lot the first half of the year. Also remember that the challenge is set up for people that actually plan to re-qualify for real. If you plan on making the cut, why not challenge sooner?