Could the Catholic Church have done more to quash it from the outset?
How did the Protestant rulers of the Thirty Years' War come to be Protestant?
Would the average Catholic ruler of the time (i.e. no more pious than any other) have considered Protestant rulers to be anathema or would they have not cared?
How was France's intervention in the war on the side of the Protestants received domestically and by the Catholic Church? Were there any religious repercussions for France's rulers (civil unrest or threats of excommunication)? Were France's presumably Catholic nobility generally supportive of this decision?
It's been a while and no one has answered you yet, which surprises me considering the argument, so I'm going to try, even though I don't feel like I can give a complete answer to all your questions. If someone comes and answers more accurately, I'll probably delete this post, but meanwhile, I can at least answer one of your questions.
Could the Catholic Church have done more to quash it from the outset?
You have to keep in mind that the idea that the Church needed a reformatio, a series of reforms that brought it closer to Jesus Christ's message, wasn't something new. It had already been circulating since the XIV century, as for example Jan Hus criticized the excessive richness of the Church (and got burned at the stake for it). Shortly before Luther, Erasmus of Rotterdam criticized the corruption of the church in his Praise of Folly. So when the higher ups of the Church heard about this unknown monk and his 95 theses, they didn't worry too much about it - he would have been brought to sense, or the Inquisition would have dealt with him. Also around the same time, there was the matter of Charles V's crowning as emperor that made everything else a lot less pressing.
So briefly, this is why it took until 1520 to declare Luther's thesis heretical, and until 1521 to excommunicate him; meanwhile, he was able to translate his works in German, making it understandable to a lot more people, as it was a much more common language than Latin. After that, there were still attempts at a reconciliation; after all even high ranking members (like for example Gasparo Contarini, that would be the papal legate at the diet of Regensburg in 1541, the last attempt to reach a reconciliation) of the Church shared some of Luther's points of view on the need for a reformation. But their attempts to find some common ground didn't end well.
How did the Protestant rulers of the Thirty Years' War come to be Protestant?
Well, Luther criticized the authority of the Pope, the legitimacy of the Church's taxes, and the temporal power of the Church. This meant that a lot of rulers saw this as a chance to slip away from the Pope's control, and to take control of many of the Church's possessions on their lands. So while some maybe actually believed in it, most had more 'real' reasons behind it.