Projectile Offset Issue

you sir..... are a genius

That is correct, projectiles do not mirror. 

Test your projectile in Scene view by dragging it around:

• If trail follows behind it correctly, then you're fine.

• If the trail does not follow behind and stays in set orientation, either modify particle effect so it follows behind or you'll need to modify some code to get it working.

Looking at ProjectileMoveScript.cs, I think adding:

transform.localScale = new Vector3(transform.localScale.x * -mirror, transform.localScale.y, transform.localScale.z);

// below line is in case angled projectiles don't mirror correctly
transform.localEulerAngles = new Vector3(transform.localEulerAngles.x, transform.localEulerAngles.y, transform.localEulerAngles.z * -mirror);

in the Start() method just after the transform.Translate() line should work.  But depending on how your projectile is generating its effect, this might not be enough for it to look right.  Yours looks to be 2D, so this might work for you.  Just make sure the pivot of the projectile object is the middle of the ball part and not the flame part.

EDIT: added line for angled projectiles.

Ah, damn.  OK, your best bet is to create your projectile as a particle effect that is independent of direction.  Look into Unity tutorials on how smoke trails and jet engine boosters are done using the Shuriken particle system, I think that's a good basis to start with and should translate easily to creating basic projectile.

It won't look like your 2D one though, but will work nicely in any direction.