2. At level 50 on fleet there will be a quest NPC that starts the line. Once you complete it once you can use your legacy to buy HK for other characters.
3. Depends on the situation. Usually conversations with NPCs you only have one companion available. If that companion happens to be on a crew skill mission then your conversation choices do not affect him/her/it. BUT there are several conversations in class stories where your whole crew is present (usually around the ship holoterminal). In these cases a companion being on a crew skill mission is moot, the companion is affectted by the conversation choices.
4. most definitely.