It seems to me, therefore, that since the title continues to be used by the early Christians, they must not have understood Matt. 23:9-11 as forbidding its usage.
Hmmm....interesting but I can't say I agree. The early Christians seemed to quickly forget many of the things Jesus taught them and in some respects, they went right back to many of the attitudes that the Pharisees had. For example, Jesus ate with tax collectors and prostitutes. The Pharisees, as we know, wouldn't even speak to such people and they condemned Jesus for associating with these people. Jesus drew the young children to him when his disciples tried shooing them away. Jesus had a very liberal attitude towards women---some of his followers, most notably Paul, did not.
I think by the time the early congregations were formed, many of the things Jesus said and emphasized were ignored. The context of Matthew 23: 5-11 clearly shows that Jesus was discussing the prominence and position that the Pharisees got off on and He was specifically telling his followers not to be like that and that attitude was not one that should be amongst his followers: "Do not be called Rabbi, for one is YOUR teacher, whereas all YOU are brothers. 9 Moreover, do not call anyone YOUR father on earth, for one is YOUR Father, the heavenly One. 10 Neither be called ‘leaders,’ for YOUR Leader is one, the Christ. 11 But the greatest one among YOU must be YOUR minister."