1. Yes, you wouldn't need to try to buy-back your status until it is expiring (or just expired.)

3. It is hard to answer for you whether pushing for Plat now or not is worth it. The answer is it depends on how much it will cost you, how much "benefit" you will receive from it and what you value. If you aren't staying much at Marriott hotels over the next couple of years, then I'd say the benefits might be limited as you won't be in the hotels to get those benefits. Also, on those "few" stays you will have, will they be FS or non-FS hotels? If non-FS, then the benefits during those stays will not be as pronounced (as they don't usually have a lounge to give you access to and upgrades can be a bit more limited as well.)