A short answer detailing whether Germany were right to sign the Treat of Versailles alone
Can we justify that Germany should not have been forced to sign the Treaty of Versailles alone?
The Treaty of Versailles was signed by Germany following the armistice which brought World War One to a close. Although Germany was the only signatory from the defeated powers, it did so on behalf of herself and her allies; so the notion that Germany was held solely liable for the War is not in fact true.
Peace treaties, such as the Treaty of Versailles, are generally imposed by the victors on the surrendered nation(s) and so there was no question that the victorious nations such as Great Britain, France or the USA would be a signatory. Russia had also ceased their involvement in the War in early 1918 with the signing of the Treaty of Brest-Litovsk.
Austro-Hungary is generally held to be almost as, if not equally, responsible as Germany for the escalation of hostilities and the outbreak of the War following the assassination of the Archduke Franz-Ferdinand in summer 1914. Germany backed Austro-Hungary and effectively wrote Vienna a blank cheque to go to war with, and annex, Serbia. Vienna gave Serbia an ultimatum, which was never even conceivably going to be accepted, or they would declare war. This was despite the fact that this would inevitably lead to Russia declaring war on Austro-Hungary, and then Germany and France entering the War on their respective sides due to established military alliances.
However, one of the key reasons that Austro-Hungary was not a signatory to the Versailles Treaty in 1919 was that the Austro-Hungarian Empire had in fact been dissolved on 31st October 1918. The separate Republics of Austria and Hungary concluded separate Treaties with the victorious nations where they lost 60% and 72% respectively of their territories to smaller independent nations. Germany, on the other hand, was still the same sovereign state in the aftermath of the War and so the Versailles Treaty could be applied directly.
Another reason Germany was the sole signatory to the Versailles Treaty was for the purposes of reparations. The Treaty was not signed on the basis of an indemnity, as many peace treaties were in the centuries preceding the War. Instead, it was designed on the basis of reparations, or compensation based on the damage caused on the Western Front to France and Belgium in particular. The German army was the largest force for the axis powers during the War on the Western Front and so Germany was held predominantly liable for the compensation held to be due. Other axis powers such as Austro-Hungary, the Ottoman Empire and Bulgaria concentrated their forces predominantly on the Eastern Front, the Balkans and the Middle-East; these theatres of war were not the subject of the Versailles Treaty though.
Therefore, that Germany was the only signatory for the axis powers to the Versailles Treaty is logical and it would not appear justified to have had another signatory.