Looks fairly standard of the stuff that you get delivered, rough with the smooth and all that. Unless you take the time to go and sort out which bits of timber you want from the supplier. Maybe next time upload even more photos.
Normally , or at least in the past (20 years or so), there would be a coax cable fitted from the living area up to the loft, then the new owner is expected to fit an aerial.
It will be heavy so might be helpful to have an assistant. But have you actually checked if taking the door off will be enough? How would taking the handle off help? Surely the pinch point is when then door is open - the gap between the open door on the left and frame on the right?
AFAIK you need to pop that cap off (1.47), then undo the four screws (2.29) for each hinge.
Unless @crank39 or @ronniecabers know of an easier/better way.
It doesn't always have to, if the setting out permits it then a smaller stone on each end is fine too - subject to the copings on the end not being adequate. But that's irrelevant here in anycase.
Without knowing the length of the run and size of the stones your only other choice in that situation (apart from having a 200 slither at one end) would be to cut every coping stone down a bit. Got a photo?
Think your terminology is wrong there, underlay is the roof membrane, which is correctly laid over the top of the eaves tray. What is lacking is the tilting fillet, the solid timber wedge shape bit of timber that would sit under the eaves tray/membrane and prevent this sag. Obviously the battens...
Well he's partially right that many older roofs do not have an underlay and also do not leak.
However there ought to be a continuous tilting fillet to prevent the sag you're seeing. You say he's fitted the underlay beneath the eaves tray but we can see the underlay on top?