There are commands that /targetenemy already. But they will get anything that you can attack, like a hunter's pet, totem, etc. The enemyplayer part is so you only get players not other stuff also. You want a castsequence for multiple spells in a macro, try:
/targetenemyplayer
/castsequence reset=target/combat Shadow Step, Cheap Shot

You can heal anyrandom player (no guarantee who you get if you just /targetfriendplayer), but I find it more useful to heal whoever the mob is beating on. I foucs my main and then my alts use:
/cast [target=focus-target-target] Chain Heal

No, you can not target by raid icon, at all, anyway at all.