Yes, the C is more important than the first B, but as you said the second B is the most important of the 3 notes. Therefore, as you say, the stress should be "More Less Most" respectively, which I agree with, but you can get a very decent accent out of an up bow and then you have the bow ready for the stronger accent, using a down bow. You are essentially, "saving the best for last." So, an up bow, which can be very strong, is used, and then the second note of the three, or the first B, will decay from the C continuing on the up bow. Then for the most important of the three notes, a down bow will be used, which is appropriate, being the strongest bowstroke.
The dissonance can be brought out, but what happens when it is done with a down bow first is the C is made the most important of the three, when it should be the second most important. The stress is then "Most Less More," and the chord tone is then overshadowed by the emphasized appogiatura, which is just that, a grace note. The listener then interprets the first note as the most important.
As for the comment about it being commonly played on a down bow by all of the other instruments, that is up to the editor. Mozart put no bow stroke markings in his scores to my knowledge, so the editor is the one who puts them in. So we are left with what musically makes sense. Classical interpretation denotes that pick up notes should be performed with an up bow. If there are no markings saying otherwise in Mozart's hand, then there is no reason to assume otherwise.
If you are going for a modern interpretation, which is honorable, but not accurate, then starting on a down bow is fine. Then the dissonance is emphasized. If you are going for a classical interpretation, then starting on an up bow would be best. Then the dissonance is touched upon, and it functions as an appogiatura and anticipation of the chord tone.
Just my take on things, and I don't mean to sound as if I'm speaking sternly or angrily, that's just how I sound when I discuss things, haha.
