Look, don't bother with the focus thing. Do you need to have a variable main? If so, set up FTL keymaps and make an assist macro that can listen to FTL keystrings. If not, put an /assist MyMain line in your autoshot macro and call it a day. For example:

Code:
#showtooltip
/assist myAwesomeTank
/startattack
Actually, you might want to switch your main from just an attack button to:

Code:
#show Attack
/startattack
Or whatever. You can also use bangs (i.e. stick an exclamation point in front of) things like "/cast !Attack" or "/cast !Auto Shot" when you're dealing with toggleable abilities. This will keep it from turning it off next time you hit the button (only in the case of what was happening with your main though - the follower's problem was as I outlined earlier, you need a target to autoattack with Attack / Auto Shot / Shoot).