1) yes
2)Yes, many interline agreements worldwide
3) If youre parents or partner are listed on your staff travel, they can buy as many ID90 as they want and travel without you.
4) You cant take friends with you at all unless they are listedas a travel companion, and then you have to travel with them.
You can get them slightly discounted tickets called HOTLINE, not a great deal though.
5) if you are flight crew you get a free club confirmed ticket each year, after 7 you get another free sby with first entitlement. If your're cabin crew you get a free sby ticket after 7 years.