The Arab sabotage you mentioned happened for a decade plus to the Ottomans. The reason it became a problem in WW1 is the Ottomans had almost no troops in the area due to other war time losses. If you have another 1-2 divisions in Arabia in WW1, the Arab tribesmen are not an issue. In may ways the resemble the train robbers of the American West. These bandits simply did not operate often near significant numbers of armed troops or police. The same will be true in Libya.
There are plenty of rails available in Europe. It might require some changes to production schedules or purchases from Germany, but this is not a serious issue. The Ottomans managed to build RR without being able to produce or import significant quantities of rails. So too the Italians will be able to find the rails.
As to your comment about the RAF, yes, the British counter efforts will help limit the effectiveness, but it will still be a huge net benefit to the Italians. And it is also largely self correcting as the Italians have additional success. As the Italians move close to the Nile, and maybe even take Cairo, the combat effectiveness of the RAF in Tobruk and surrounding waters will be limited. And to be fair, all the problems the Italians have will slowly develop for the British. At some point, the ports in the Med will have to be abandoned. At some point, supplying Cyprus will be extraordinarily hard. The British will have to unload in ports on the Red Sea. The rail lines will be under increasing levels of interdiction. IOTL, western Egypt was the point where there is a natural defensive line that tends to form due to logistics. ITTL and after the use of the Med Sea ports stops for the British, the natural defensive line will be not too far from Red Sea or the Suez.