The Painting at San Clemente Basilica

This painting, found in the tomb of St. Clement in Rome, depicts Ss. Cyril (Constantine the Philosopher) and Methodius presenting the relics of St. Clement to Pope Adrian II - wrongly identified in the Latin inscription as his predecessor Nicholas.  Two sections of the image have been defaced - whatever was held between the two saints on the left, and whatever was underneath the Pope's right hand.

Now, it is well known that Cyril and Methodius had another reason for their visit to Rome in December of 868.  In addition to returning the relics, which Cyril had discovered during a mission to the Crimea, they secured the Pope's endorsement of the Slavonic liturgy.  According to the Life of Constantine by their star pupil Kliment Ohridski, "the Pope, having received the Slavic books, consecrated them and placed them in the church of the Virgin Mary".  Later, Kliment goes on to tell that during Cyril and Methodius' stay in Rome, they and their followers "sang all night, praising the Lord in the Slavonic tongue."  Another work attributed to Kliment, the "Italian Legend", describes how Adrian "ordained [Constantine's] brother Methodius bishop, and others of their disciples priests and deacons."

Later Popes did not all share this enthusiasm for the Slavonic books and an autonomous Slavic clergy.  Could it be that the defacement of this painting reflects an effort by someone in the Roman church in later years to destroy evidence that a Pope had not only approved the Slavonic books, but also personally met with and ordained the Slavic-speaking clergymen who were later persecuted by the Frankish faction of the church in Moravia?  

I would love to hear from anybody who knows more than me about this relatively obscure episode of medieval history.  I'm particularly curious about the crowd of figures surrounding the saints.  Are these generally thought to be Roman clergy or Cyril and Methodius' students?  Maybe some of both?  Also, it appears that St. Cyril is shown with dark hair but a white beard.  Could this be another alteration, and what is the purpose of it?