eaves protection and eaves support tray

you don't say if the roof is under felted. your under felt usually starts in the gutter is tacked onto your fascia and leads up the roof. any water that then gets under your slates runs down the felt into your gutter and away into the drainage system. because the felt into the gutter is exposed this rots allowing water ingress into your fascias which rots them from behind. when you remove the fascia the fist row of tiles are usually removed to allow access to the under felt , this is cut at the fascia board and felt support trays installed to replace the missing felt leading into the gutter. on a slate roof it is a lot harder to do because your slates are double slated on the bottom row and each slate is double nailed. you will lose a fair few slates and scaffolding will be required to install drip guards ,eaves protectors,felt support trays. as for your third question without actually looking at the job it is impossible to say what is and what is not required. although providing the dormer roof is running towards the front of the dormer guttering will run the water down onto your roof tiles rather than down the window.