You'd have to add it up so that the days on the ML roster won't tick ML Service over to 1 by the end of next season, but sure. That's about right.
Typically, yes, but some have seen one. I've seen several promotion "bumps" during the playoffs, so it's entirely possible.
The safest time, imho, to demote is before ST the following season, but you'll be stuck paying a second year ML minimum salary (~$343k).