Well, how can you have Mozart and Sibelius together unless they are in your CD collection? Apart from arranging Mozart in the style of Sibelius? That would be rather grand, but I don't intend to go there, too much music of my own to write in too little time, BUT I have heard it speculated about!
What would Mozart have done with sibelius software? Do you know, I don't think Mozart needed it. I think that Mozart had one of those Asperger genius type brains that consider all possibilities simultaneously. One of the type of genius brains that could become a grandmaster at chess, or tell you without thinking for longer than 15 seconds not only what day of the week Elizabeth the First was born on, but precisely how many hours elapsed between her birth and Elizabeth the Second's!
Wouldn't it be something to have a brain like that? Well, yes, but it would be so very, very noisy! I know when I compose (with sibelius) the themes bounce around for days afterwards and I can't think or sleep straight. Think what that would have been for Mozart! That is some internal orchestra!
All the best from Me!