A couple of comments since I teach a person who regularly 100 miles three horses (trak x arab). Is the horse really upside down? Or is the horse just looking at a distance? The topline is quite nice, but the horse is likely tired and looking far away, so the underneck is out slightly. However, the horse is gather behind, and fairly balanced.
With my students endurance horses we uses dressage theory as a basis for figures/balance. Merely by allowing a lower(ing) of the neck, it should help relax the under neck. If this does not ‘work’ then work on positioning (laterally, on circles etc) will help the horse to learn to seek forward/down/out. This then makes a great impact on the ability of the horse to swing through the back and take longer slower strides (which makes the heartrate/etc lower). This students horse always wins conditioning.
A comment (which is problematic with arabs), the saddle could be a smidge more back, particularly if its treeless. Treeless saddles MUST be placed three fingers behind the bony back edge of the shoulder blade, which is almost impossible on shortback/arabtype horses.
Do not attempt to get a distance horse rounder per se. You want such a horse longer, but still out to the bridle. By asking the horse to mobilize the jaw, chew, and allowing it, the horse will relax and go longer. Slowly, methodically, without gadgets. You do not want the jaw to yield per se, you want the horse to chew and then allow the streching. It is not giving you want to engender, it is the ability to follow the hand. The only tossing of a head that might happen is if the rider is clumsy/sustained too long in their request. Equally there is no reason not to pulse the horse out on a circle (a form of leg yeilding), but again ask/relax/ask/relax. Aids are applied for a step, not strides, and not holding to force yeilding.