Independant, "the Vatican had to obtain special permission for him to be allowed to enter the pocket statelet." I am not sure that that is quite so.
The Holy See still talks to the Mugabe regime and rather than issuing specific invitations there is general invitation to all world leaders, or their representatives, to attend such events. Mugabe has decided to go himself.
Apparently, from whichever Italian airport he arrives at he is under Vatican protection from there until enters the City State and so is immune from arrest by the Italian state in the same way he would be if he arrived in New York to attend a UN meeting.
I am pretty certain I know what John Paul would have said to the President, Pope Benedict is less outspoken.