Hi
To answer question 4 first, as far as i can tell, the answer is no, you can't mimic Divisi with the Special Editions. you can stack instruments in slots (this is very effective for unison lines etc) but as far as i can tell by tweeking with it they dont divide (for exapmle, 14 violins into 2 groups of 7) to play two notes on the same stave....thats what Dimension strings are for, as with the sound set installed will behave as you'd expect with a2 etc.
As far as i can tell, Tremolo works as expected, i havnt experianced any negative playback at all, either using 'tremolo' as a text instruction, or using dashed notes directly in the stave.
The other questions i cannot answer as i only have SE1 and 1plus at the moment, but what i can say is that the integration with Vienna and Sibelius is wonderful, so much so that i cant seem to get into using a DAW (Logic) as i'm so used to how Sibelius works now.
That being said, logic works really well with the special editions now as well with a plugin, so if thats your daw of choice your covered.
If you havent bit the bullet yet....treat yourself and get it....you'll be glad you did!
Good luck mate
Kindest
Paul