Is the ḥadīth of 'Uthman ibn Abi al-'As about the Muslim cities and the emergence of the Dajjal proven to be from the Prophet (peace be upon him)? And does it have meaning if it is not authentic? And have there been statements from the Salaf concerning the parents of the Dajjal other than the ḥadīths of Ibn Sayyad? And is the description of the Dajjal's eye as being like a sputum on a wall authentic?

The Antichrist (Dajjal) is a tremendous tribulation. His description has been mentioned in authentic and weak hadiths, and the hadith you mentioned has a weak chain of narration. Some phrases in the hadith have authentic corroborating evidence, such as the presence of seventy thousand Jews from Isfahan with the Dajjal, the descent of Jesus son of Mary (peace be upon him) and his killing of the Dajjal, and the speaking of rocks and trees.

If the hadith were authentic, its meaning would be that Muslims would have three major cities (amsar), and the Dajjal would emerge, defeat the army of the East, and be accompanied by 70,000 Jews wearing green cloaks (tayalisa). Most of his followers would be women. The Dajjal would enter these cities, causing division among their inhabitants. Muslims would gather at the Aqabat Afiq and suffer severe hunger. Then Jesus (peace be upon him) would descend, and their leader would lead them in prayer. Afterward, he would kill the Dajjal with his spear, and the rocks and trees would speak to expose his Jewish followers.

As for the Dajjal's parents, nothing reliably attributed has been reported about them except what came concerning Ibn Sayyad.

The description of the Dajjal's eye mentioned in the authentic hadiths is that it is "as if it were a floating grape," or "his eye is effaced with a thick growth over it," or "he is blind in the right eye," or "his eye is effaced." All these descriptions indicate a defect in his eye.