"I don't even know if the suns light produces a noticable effect or not. . . are the orbital calculation for them any different than for an astroid?" "
I don't believe so- though not very familar with comet orbital calcuations. I imagine they are not very precise. I don't think any calculations on most objects are precise enough to determine a definite hit or miss of earth.
..."in any case if it wasn't negigible then perhaps you could instead shield the sun from the comet. . . you could nullify the suns effects and over a long time it will have retain enough energy to wix by the earth ahead of time and nt hit it. . . ."

The sun's energy will gradually heat the object as it comes near. In order for a rocket (or something behaving as a rocket) to be efficent the propellent must have high velocity.

"of course this would have to be BIG and you'd have a heck of a time trying to keep it in front of the comet. . . in fact I think the size of such a shield would be extremely prohibitive."

I considered this briefly.
Also:
I think using reflected light (more intense nearer to Sun) would be better than shading but neither seem very practical at this point in time.
-gb