You should be able to spot patch without major difficulties. Just be sure to remove any loose or powdery mortar from the original layer. It's also important to moisten the old mortar and the underlying brick in the areas you're patching in order to achieve a strong bond between old and new.

This is exactly what I'm working on (except no brick underneath), but I'm also dealing with areas I'd like to just skim in the inner hearth. So I'm wondering about a product that can fill 1" deep holes and also just skim and that will bond well. I will have some areas as thin as 1/8 or less.

Rutland fireplace mortar will do the trick. Less than $10 for a caulking gun tube. Clean area well and wet with water first.

I have the same exact problem. I was thinking of getting a slate piece big enough to lay it on the fireplace floor. Would that work? If not, who would fix something like that?