Hi Ralph and Belinda and welcome to the group. As Zildjian (zildjian) has noted that's a pretty formidable shopping list you've got there and difficult to match in a pop-top particularly as you're already committed to a double-cab pickup - an extended-cab, or even a single, would certainly open up more options.
As you've remarked the Gazell is a very elegant and sleek design but I suspect the private toilet space might be impossible to achieve in it. Virtually all demountable manufacturers build specifically to order but, as a general rule, those that manufacture in fibreglass (like Gazell and many others) are less likely to be able to accommodate individual modifications and alterations simply because their building process is based on existing and expensive moulds. If you're going to France anyway to look at the Gazell, have a look as well at RRCab.
http://www.rrcab.com/ Their sandwich construction method results in a much squarer box but may offer more flexibility internally.
The model that Zildjian included a picture of in his post of the 24th is Italian and is available from Vacanze in Fuoristrada (ViF). ViF are actually a camper hire and tour company but seem to be the only source of this camper which I believe is manufactured for them in Iceland. I think it does ✔ quite a lot of your boxes but is actually built for an extended cab. I'm afraid I don't know whether or not it would fit on a double cab. ViF are at
http://www.vacanzeinfuoristrada.it/index.html.
If you were venturing as far as Italy though it would also be worth looking at the new-ish pop-top version of the 'Dragonfly' from Overcar
http://overcar.it/cellule_abitative.asp. This is made for double-cab pickups and I think fulfills many of your criteria, including a toilet and shower area, although in its standard form privacy is only provided by a curtain.
Also in Italy, Modulidea build (and custom build) an extensive range of pop-top campers both in fibreglass and in polyester sandwich. They are a long-established and well-reputed builder of expedition and adventure vehicles. They are at
http://www.modulidea.it/2012/default.asp.
Finally, if you're able and willing to go looking even further afield, Truck Camper Europe in Spain coach build an all-aluminium camper which can be fitted out entirely to your own spec. That may well be going too far or too ambitious but I mention it simply because the basic and empty camper shell at 4900€ +VAT is so competitively priced that it might be a viable option.
https://sites.google.com/site/truckcampereurope/inicio
Apologies for having gone on at length, I hope some of it may be useful. Good luck in your search.