Let me say that if those granite blocks are really that massive I would tear them out for free just to keep them.
Granite is very expensive and blocks are used for outside stairways, etc so it really doesn't matter if one side gets damaged. Maybe you could offer this kind of deal to one of your local stone dealers.
No, I'm not a strapping fella, this is what employees are for. My suggestion is if you can't have the stones removed then install a portland cement based floor leveler over the hearth of the fireplace and wall float over the vertical surfaces. Screed it out smooth and flat with a cheap 2' alluminum level. Let dry for 24 hours, then you may tile over it or something else.
If the directions on the levelers allow you to add the acrylic latex additive then I would do so.
Plaster will probably crack over time and doesn't handle moisture well (not that there will be any but you never know). Any finish over the plaster (or leveler) may not be heat resistant so make sure you check befor proceeding.