On the 787, pets and other live animals can be carried in either the bulk or forward cargo holds, both of which are heated and air-conditioned. The bulk hold is the one where last-minute bags, mail, etc., are loaded, that don’t go into those metal baggage containers. KLM has found that, on their 787s, the temperature control for the bulk compartment is not reliable enough to accept pets there. This seems to be a fleet-wide problem, and I don’t expect there to be a fix for this anytime soon. Which leaves the forward hold, but on KLM flights these are often full with pharmaceutical or food products, which need to be cooled to 5 deg C instead of the 18 deg C that pets normally require (although if your retriever is anything like mine was, she would love after her time in the desert!). KLM has decided not to take any pets at all then, in order to be able to accept perishable cargo in the forward hold always, which produces way more revenue than animals. Other airlines may be willing to turn up the temp in the forward hold for your pets, or find that the variation in the bulk hold is acceptable, so I wouldn’t worry. If you do end up on a 787, however, and this goes for any airplane, really, a gentle reminder to the crew that your pet is on board, with the request that they keep an eye on the temperature is always a good idea. When I see on my paperwork that we have a pet in the hold, I will usually ask the owner what temperature they would like, as I know it varies greatly from animal to animal, and set it accordingly.
As for flights, I know you wrote that DL isn’t taking checked-in pets, but is that maybe only on regional aircraft, because I don’t see that on their website. It seems that you could fly either AUH-AMS-JFK-LAX or AUH-AMS-MSP-LAX on Friday the 29th, for example, and always be on an Airbus or 767. Just a thought.
Good luck bringing them home.