Overwatch Leavers Penalty – How Rejoining the Same Game Affects It

Not sure about console but on PC at the top of your friends list there is a 'Recent players' tab.

If you right click on a person you will see an 'Add friend' option in the dialog menu which, provided that they accept the request, adds them to your battle.net friends list.

You can also right click on a person's name in chat and add them that way (as pointed out by GnomeSlice below).

Yes*

If you walk into the direct line of fire between McCree and one of his locked in targets, you will intercept the bullet meant for the person behind you.

McCree's Deadeye is a hitscan attack that charges at a rate of 170 damage per second.

In this video at 0:11, you can see a Roadhog with full lock (600+ damage) survive Deadeye because a Reinhardt walks in front.

While this does not directly prove the point, as Reinhardt gets one-shot by the 600 damage bullet meant for Roadhog, and only one bullet is fired, if Reinhardt had spent any more time in front of Roadhog, a second lock on would start for the Reinhardt, and both would hit him.

I'll back this up with a .gif showing this when I return from work.