Ugh...dinner bell rang whilst posting my answers before....
3. No, in fact an IC may not leave a unit after arriving with it via
Reserves. It may leave the unit in subsequent turns.
BRB FAQ pg 3, second question.
4. This is covered in the rules for Terminator Armor in the SW codex on pg 61. They take up two slots and may not ride in a Rhino or Razorback.
5. Can't help you as I do not understand the question, please try to use actual game terms for clarity.
Firing an Ordnance Barrage weapon directly you do subtract the
Ballistic Skill from the scatter distance rolled.