Sniping
If you've already successfully used Stealth at least 10 feet from your target, you can make one ranged attack and then immediately use Stealth again. You take a –20 penalty on your Stealth check to maintain your obscured location.
Casting Magic Missile is an attack, even though you don't roll for one.
MM = metamagic?
Nothing, as you'll still be noticed if they can see you. Even if you have still, silent, and eschew materials, it's still fairly obvious that you're casting a spell.
All you need is to not be seen when you cast it. Have your back turned, keep your hands hidden under your robe, or be around a corner/behind some large wall or object.
You may also want to find a way of concealing your material components, if any.
I've done exactly the things you describe in this manner to great success. A word of warning though, kings and other stately people often have wards and/or court magicians to protect them from such things.
The intent wasn't an attack but enchantment. The idea of being about to cast charm spells without anyone being the wiser (including the target) sounds very cool. Example being in a Kings court, being in front of him, casting a suggestion no one else notices.
When casting enchantments i roleplay that the verbal and somatic components that are needed are disguised as the act of convincing (like on a command spell the verbal components are part of the phrase you put the command in, or showing the palms of both empty hands in act of submission for somatic components) so someone who has spellcraft knows this and is always ready to analize that gestures, someone who don't can't recognize it as such if he doesn't know the magic user is capable of doing so. If you haven't studied magic, you recognize it by its effects and nothing else.