You are right, that is a Gouldi/flavi. Looks like it was from Python Pete. And, V.p.horni, is from Indo and V.gouldi is from Australia. V.gouldi, is not a desert species. V.p.panoptes, is also not a desert species. Actually, V.flavirufus, V.panoptes, and V.gouldi, can and do occur in the same exact spot.
But I really wonder what that has to do with keeping it as a desert species? I even wonder what the difference is in captivity. I bet, if you took that nice cross and kept it in your water monitors(indo) cage, it would do great. Heck, I bet if you kept your water monitors in the cross cage, they would do well too, as long as theres a water bowl with water and the cages are big enough.
I get the feeling the difference between a water and a Flavi in nature is more like this. A water spends much of the time trying to stay dry(drying out in the sun) and a flavi spends much of the time trying to stay humid.(in humid burrows) other then that and the knowledge(ability) to find food(no concern in captivity) there is very little difference. I think the actual needs are exactly the same. While the tolerances my be slighly different. For instance, a flavi can go for a longer time without any water. But in captivity, you should provide water to both a flavi and a salvator. Now, if you could keep a Flavi without ever giving it water, then you may have come close to keeping it naturally.
Actually what does it matter what label you put on how you keep it, the monitor(and your child) look like they are in great condition, as your captives always look. I would(of course) be more concerned on how well the monitor is doing, then what you call your cage(which is never,desert,indo,african, asian,etc) or where the monitor came from.
Keep up the good work and when are you going to allow some breeding? Cheers FR