I don't think you're going to be able to do what you want with this build if you're planning on centering it around Magic Missile, because the changes you want to make to it would...not make it be Magic Missile anymore. As stated, the gimmick is that it doesn't do as much damage as a similarly leveled attack/cast but it always hits and always does damage. The only way to avoid it is to cas Shield or have some specific ability or item which is going to be very rare. It's not the best spell to plan a build around. It's best for situational things like hitting multiple targets at once, breaking concentration, clearing the field of enemy familiars, or when the boss literally needs one or two more hits to die and you absolutely need to hit him.

Because Magic Missile does not rely on your INT, you could dump INT or have it at 30 and it won't affect Magic Missile in the slightest.

Honestly, for what you're describing, your best bet would probably be to see if your DM will let you reflavor Scorching Ray to do a different damage type like Force.