Again, I agree, with minor qualifications.
By "best possible," I mean an environment that will bring out the very best in the player, which is often determined by a combination of factors (as you mention), not just level of competition. And I agree with your profile of the successful player as someone who naturally loves to do those things that will be beneficial to level of play--most of the things you mention have to do with individual effort rather than club environment, which is where SF started this thread way back when.
I also know that passion for the game can be nurtured, and that it can also be burned out of a player; I've seen girls in highly competitive environments who somehow lose some of that spark, and what used to be a passion somehow turns into a job. I've also seen players who have previously been more indifferent develop a love and intensity for the game. There is an X-factor in there that goes beyond difficulty of competition level, and I think wherever the player can find that positive factor mated WITH competition, that is where she will find the "best possible" situation. Different things can often click with different players, so everyone's most effective environment might not be in the same spot.