Stoner, I think all the work you have put into the landie deserves a bit better than the roller & hammerite.
Takisawa speaks sense about being able to retreive even the worst of painting but its time consuming & not even sure you will get the hammerite to cut back properly it also takes an age to get properly hard, keep the hammerite for towbars & the like.
Stick with proper 2k paint, 800 grit finish is the roughest you want to spray top coat onto, let it dry, really dry then you can 1000 the rough bits where your spraying didnt go quite to plan, 1200 the rest & you should be able to go straight to a polish, maybe a little T cutting before hand.
Your white idea sounds good, hides quite a lot of sins, however again after all that work & the stripped down condition you have it too now, why not just do the lot.