The initial projections for the completion of the railway from Constantinople to Baghdad were pretty optimistic, both in terms of costs and completion times. It is anyway highly unlikely that a project of such magnitude in a completely undeveloped area of the Ottoman empire and with major mountains to cross could stay on schedule.
I am willing to take some suspension from disbelief and grant you an early completion of the link to Baghdad. This would have certainly changed the operations in Mesopotamia although it is by no means sure that the Ottomans could have foiled the British landing in Basra. A raid against the oil fields and the Abadan refinery would also be probably feasible: it would be a slap on the face of the British but the impact on the war would not have been major.
The problem with the operations against the Suez canal would not have been reduced by a lot. There were problems with the supplies of ammunition, artillery, food (ammunition had to come from Germany but with Serbia still in the fight the Belgrade route was out and neutral Romania was making difficult to send war materials through Bucharest). A successful attack on the canal would have required prefabricated sections of bridging pontoons to be carried by the attacking army. Kress von Kressenstein (the German officer advising Djemal on the campaign) wrote in his memoirs that it would have been necessary to build a railway from Ma'an to the canal to mount a fully supplied offensive with sufficient strength to break through. He also added that the construction of the railway would have taken 3 years (and he may have been optimistic). With no railway the strength of the attackers had to be drastically reduced: from the 50,000 men that von Kressenstein deemed necessary went down to a nominal strength of 20,000 (but only 12-13,000 reached the canal).
Procuring camels proved to be a hard (and very expensive) task. Getting on board Bedouin irregulars much more of the same: the Bedouins swore to participate in the Holy War, took very hefty bribes in gold and disappeared in the desert.
Given these problems it was certainly impressive to get 12,000 men across the desert, but the likelihood of a successful crossing of the canal was not very high. The British had some 35,000 men protecting the canal with good logistics, entrenchments and barbed wire barriers (not to mention the added protection of some warships in the canal).
The only reasonable chance of success would have been an anti-British insurrection in Egypt and Sudan but notwithstanding the efforts (and once again the gold) that the Germans poured into this attempt the insurrection never materialized. Even Abbas Hilmi II (the Khedive deposed by the British in August 1914) who was in Constantinople when the war broke out was a dubious card for the Ottomans: Enver Pasha was convinced that he was still dealing with the British (and Abbas was sure that the Ottomans had no intention to put him again on the throne).
BTW, the number of Germans troops you assume might be transferred to the Middle East (500,000!!) is wrong by at least an order of magnitude. Even if it were possible to transfer so many troops (and it wasn't) they had to stay on the western and eastern fronts, and stiffen the Austrians on the Italian front.
What the Ottomans needed was artillery and logistics specialists and as much supplies as the Germans could send.