The missile projectile doesn't appear when set to normal. As for the make unit immune, you can do the following thing:
- go to your Trueshot aura and go to "targets allowed", mark a thing like non-ancient and go to your unit and go to unit classification, mark ancient. So it you say: the aura effects range units but it can't effect ancients.
You can change the Trueshot ability so it effects melee units only, range units only or effects range and melee units (like command aura).
You can use dummy units to make melee units fire missiles or edit the models to add missile animaties to the attack animation, but that's alot more difficult. If you want to go for the dummy units search in google for tutorials.
Hope I helped.