1. In principle each airline can decide for their own frequent flyers what level they give for what amount of miles. In the end they have to pay for the benefits their own members with status get from other airlines.
2. There might be special provisions within the ST contracts, which might impose restrictions on elite (plus) qualification, but I doubt anybody will know this. This is not public knowledge.
3. There is no alliance-wide upgrade priority rule within any of the 3 airline alliances.