We both think what you are doing is brilliant. Old buildings have a time scale of their own and they do dictate how you live in them, not the other way round. So, if you want the slates above the fire place showing ~ show them! After all, we have massive slate floors here, wooden partitions which are now exposed and floor boards over a foot wide which are stripped back, but when the house was built over 400 years ago, and for much of the time since, they would have been covered.
Kathy & Jon