What’s the difference between “Highest Kill Streak” and “Kill Streak Bonus”

The wording on the bonus is really silly (same goes for the Nemesis ribbon). Once you get 5 kills, you're considered to be a on Combat Efficiency streak. Your 6th kill gives you a 1 streak bonus, 7th kill gives you 2 streak bonus and 8th gives you a 3 streak bonus and hence the ribbon.

It is possible to get multiple ribbons in the one life. Once you have got the first one, (I think) you will get the next one for every 3 subsequent kills in that life.

So if you go on a 23 kill streak you are going to 6 get of these ribbons (8 + 3 + 3 + 3 + 3 + 3)

As the squad leader, you can set objectives by sighting in a target and pressing the 'back' button (left of the guide button). (You receive additional points when those orders are executed.) The easiest example would be in a game of Conquest. Point yourself at a capture point on hit the Select button. (Xbox) You can also set WayPoints, and at some point the Squad Leader can call in artillery strikes. I'm sure there are additional commands, and other methods based on the platform that you are playing on.