Dave this solves all of my dilemmas
A dedicated lamp inside the lavatory compartment helps the most.
I have looked at all the photos of roofs I can find and I cannot find one example of a coach with oil lamps set off-centre (however logical this might seem here), except beside a clerestory. Apparently, the method of handling the lamps was for someone on the ground to throw them up to someone standing on the roof, who lit them and popped them into place, and vice-versa. There were lots of breakages so they are difficult to find nowadays. Having the lamps in a row would make the task a bit easier (safer?) for the person on the roof.
The external doors to the luggage compartment are solid, no windows or droplights. I had omitted the extra door from luggage compartment to lavatory to let daylight (and oil light) spill through into the luggage compartment from outside the lavatory. If I am allowed to have a lamp in the luggage compartment then the second door makes sense. And I still need only five lamps.
All of the passenger doors except the servants' are hinged on the left and the only clash is servants'/lavatory. The lavatory door moves slightly towards the centre to make space for a wall lamp next to the obscured glass window.
The two internal doors into the luggage compartment are opposite each other in case of a lamp failure, and I have added some flooring: carpet in the saloon, linoleum in the lavatory and bare boards everywhere else. If I can make a handrail I can make a towel rail so I've drawn this in too.