I think the scenario here is that the USA fails to be drawn into this war but otherwise the cast is the same, the war is grinding out through 1916 when the Entente begin to scrape bottom for funds and manpower, so Russia is faltering, Italy wavers and the French cannot commit to yet another wave of offenses for fear of mutiny. As Russia weakens Germany chooses to break her rather than take more ground to the West, sends peace feelers that are too onerous and sends Russia into revolution while gaining the ground to free it from the dithering effect of the blockade. OE has lost enough to the UK but I suspect the Arab revolt fades if victory is not at hand, the British and French designs are becoming obvious, the Arabs waffling jump back to OE and the Middle East falls apart. The dreaded CP victory is really a massive stalemate, breaking economies, welling revolution and a Russia screw.
France gets nothing. I doubt the British can hold Mesopotamia if the Germans free troops otherwise wasted in the West, a better gain if the future oil is recognized as worth more and France finally nodding for an armistice. Yet more troops are securing Ukraine and could draw attention to the Caucasus and Baku. At most Britain demands Palestine but if they try to occupy the Islamic Holy Cities they might push all the Arabs off their bus. Even the Saud clan might try to find peace with the Caliph. Britain is loathe to withdraw and might be dislodged but I suspect they do. They are more worried about Ireland, Persia and the festering discontent from Imperial troops, salvaging the British interests in the OE might be better than hoping to steal some dirt. If France agrees to an armistice after Russia throws in the towel the UK's position crumbles, Germany can offer a withdraw from France in exchange for most anything they want, Belgium is a sticking point with Britain and giving up there gives Germany most of whatever Britain has taken, affairs in the Middle East fade to a side show as the CPs square up for a cold peace and slow rebuilding of relations as Russia catches fire and burns out of control. Here Britain may abandon all pretenses and cut a deal to back stop that fire, instantly the OE becomes a bulwark like Germany and given a blind eye to securing B-L from communism, praying they stamp it out.
That is my best case for OE, outside events undo its losses and take away the demands, the Armenian genocide is swept under the rug, debt restructuring and accommodation become the new order of the day. The OE will stumble and hobble forward through the next decades, most poor, mostly neglected, but oil will come and rather than a fractured Middle East, a multi-cultural, multi-ethnic but religiously united state has wealth to spread over the whole rather than sink it into petty little Kingdoms and backwaters suddenly paved in gold.