The performance in the war should not have been relevant, otherwise Serbia who was soundly defeated (although after a strong resistance) should not have been awarded anything at the peace table. In any case, the CPs breakthrough at Caporetto was contained by Italian forces only (the 10 Franco-British divisions which were sent to the Italian front arrived well after the front had stabilized on the Piave river, and in any case they were never used on front line but were kept in reserve near Verona, some 150 km from the front. Even in 1918 these division were kept as reserve; the only exception was a British division which participated in the repulse of the June CP offensive and fought on the Asiago plateau for the short duration of this offensive. The final offensive which broke the Austrian resistance was fought by Italian troops only.
Re. the OP question, Dalmatia should have been awarded to Italy, in accordance with the pact of London in 1915. Unfortunately it was not in the interest of the Franco-British to allow the Adriatic to become an Italian lake, which is also the main reason for the creation of Yugoslavia and for the French attempt to create a Danubian alliance which would have blocked any Italian attempt to penetrate politically and commercially in the Balkans. In order to avoid being seen as a betrayal of the pact of London, Wilson's naive position on ethnic borders was supported: it is interesting to see that neither the Slovenes (who wanted to remain with Austria) nor the Croats (who wanted to stay independent or alternatively remain in some federal form with Austria) were asked what they wanted.
This said, the Italian delegation at Versailles would have been much smarter to stay in Paris and negotiate, rather than leaving in a huff and allowing the others to dictate the terms.
Getting a LoN mandate over the Arabian coast would have been like asking for troubles: by the end of WW1, ibn Saud had already won over all of Arabia, excepting only the "kingdom of Hejaz" which was Ottoman until WW1, but administered by the Sharif of Mecca, a title which had traditionally belonged to the Hashemite family.
The "kingdom of Hejaz" was created by fiat in 1916 by the British and the new king (Sharif Hussein) became also an Entente co-belligerent against the Ottomans. By 1925, ibn Saud had defeated the Hashemites, and Saudi Arabia was born. Note that the British did not intervene, although they created new thrones for the Hashemite heirs (Jordan and Iraq, since Syria went to the French under the Sikes-Picot agreement).
This is not the place for discussing the story of ME in the inter-bellum period, but if anyone is interested I always recommend "Kingmakers: the Invention of Modern Middle East" by K. E. Meyer and S. B. Brysac
https://books.google.it/books/about/Kingmakers.html?id=hcZrEHg4sdsC&redir_esc=y&hl=en
The book was published in 2008, but it is still a must-read for anyone who wants to understand why the ME is the mess that it is.