can they?

Sure can.

Yep...expecially if you're fighting Falmers

yes they can, anything can kill them

If they continue to get hit after they've taken a knee they'll die. They also die if the damage they're taking is coming from you and not an enemy.

Yes, except Barbas & Mjoll.

Most of them go into a "downed" state, in which most creatures will ignore them for a higher-priority target (a.k.a. you). If an enemy does enough damage, they can skip the downed-state and outright kill the companion. You can also kill the companion in the downed-state, which...can happen a lot if you're not careful.

Yea followers can die. I wanted to kill mine but I took an arrow to the knee

You would be surprised at how easy they can die by your hand. Save often if you have a follower.

So much funny I can't even laugh.

Once they drop to one knee they can be killed but enemies will stop attacking them unless you're real close and/or taking area damage (like dragon fire).

The only time mine has ever been killed is from getting in the way and getting shot in the back by me.

Good.. I was worried they couldnt.