I don't know of any Archer Followers that do not also have a Melee skill, which is why they will default to it. However, I never worry about the game killing one of my followers, as it can't in most cases. I just worry about me killing them.
I have found that even when using an follower, when I snipe with my bow, they just squat beside me until an enemy notices me, then they run off.
What you could do is just improve a set of armor for the follower, and just hope they don't get killed by you when they run in.