1. It's hard to give precise instructions without seeing your example.
2. For staccato sixteenth notes, the repetitions will be triggered automatically. That's why I suggested trying the normal staccato articulation in one of the custom cells.
3. Again it's hard to give precise instructions without seeing your example or knowing exactly what you want. You have different tools to get what you want at your hand.
- Change note lengths, velocities and start positions with the Sibelius Inspector.
- Try different articulations. For example sfz, perf-rep_leg and detache have stronger attacks than sustain.
- Try to raise the Advanced/Envelope/Rel slider for the sustain articulation. This will generate smoother transitions.
- You could also try to raise the general Release fader. It can be controlled with MIDI CC23.
Sibelius commands for controlling CC23 are as follows:
~C23,63 for default value
~C23,80 for longer release time (value 80 is just an example here).
I don't see a reason to be disappointed. You have a powerful tool at hands. Just don't expect everything to sound great automatically. You will have to shape the playback to get what you want.
Vienna Symphonic Library