It's certainly a bit higher, yes, looking at the back.
This conversion is much more similar to Alu-cabs options:
Hercules Roof Conversion | Alu-Cab Global
Icarus Roof Conversion | Alu-Cab Global
Drivelodge don't appear to have beds in them, let alone a sprung base bed, or do they?
If I were to make an overlander type vehicle with a large budget, then this sort of roof with a bed upstairs would be very high on list of desirables - but the whole project will most likely remain just desirable, aka vapour.
You should still see 12V between the wire and earth, it will drop to 0 once connected to earth, as the voltage drop will then be across the bulb.
If you don't have 12V then suspect the switch as above.
A lot of people just use the brake light switch to run the pump, and find it more than adequate, I think @simonr used one like this on hid electric Freelander?
To be honest, most of the destructive rust happens from inside and along unsealed seams, with the seams properly sealed (unlike from the factory) and the cavities copiously filled with a good wax, there shouldn't be any problems, IMHO.