Tony, is greenboard the heavy stuff they use for bathrooms, etc. to attach tile to?
Was your primer water or oil based?
If water, you can just continue with a second coat of water based primer and two coats of water based paint. I'm assuming, though, that the primer was oil based from the description of the smell you describe.
Don't paint water based paint over the oil based primer, if that's what you have. It will make the sitch worse in a number of ways.
If it is oil based primer, consider putting a light coat of mud on the greenboard, and putting up 12 by 12 tile. If you have two sheets of 4 by 8 greenboard, you would need 64 tiles, and you can buy seconds for around a buck apiece, maybe less. Maybe a tile cutter for a few hours from a rental place, and a little mortar, ten pounds. You should be able to tile for around a hundred bucks.
I would scratch the dickens out of the paint so as much of the green board is showing as possible. You can get a wire brush that will attach to a drill that will make the job easier. Also, go with the thickest base coat that the tile supply place will recommend.
Be sure you get clay based finished tile -- the fired surface -- you need to seal off the smell of the paint completely and you don't want any migration through the tile. You might want to add "milk" to the mortar to help it stick better, and make the seams as tight as possible, tile to tile.
There are a couple of types of coatings you could add to the tile once it is up and dried -- I'll ask our tile guy for a suggestion of brands, etc.