Nice setup and nice to see 4Land is known here in Norway.
I had a chance to visit Tomek's workshop this summer and see his team installing such roof on few vehicles, while few/several more were there for other service. The place looks like Defender's heaven ;-) Actually, I was not the only 110 that day with Norwegian plates.
I have similar roof, different brand, installed in Germany few years ago. As it was done by previous owner, I cannot help with any information related to formal approval you discussed earlier. But I did already two EU controls in person without the issue (ekhm.... let's say - without the issues related to the roof installation ;-)
If that will be helpful for you - I managed to include the roof into the car value for the insurance, under the category of permanently installed equipment with specified value. Cost extra, but I assume it is worth in case of bad luck happens (roll-over, branch/tree fall on the roof etc). And I know in person the bad luck can happen as I lost a car in carpark fire at Sola airport 2 years ago...
Short comment about having/not having the roof. I bought the car with roof already installed, so no reason to change it, however if will be about to build it on my own
I would rather prefer the external tentbox, which I could dismount when I don't need it. However - it is nice to have it permanently, which opens possibility for a spontaneous usage. Having few colleagues who using external tentbox - they keep it permanently installed most of the year. In that case - built-in roof looks more elegant, comparing to ugly plastic box ;-) It depends a little bit if car is used daily or specifically for adventures.
Various pros and cons, depends on specific installation (e.g. I have no internal light console like 4Land's solution). I could potentially elaborate more having some practical experience, if you are interested.