It is quoted from Deut. Then verse 22 says that a slave is the Lord's freedman and a freeman is the Lord's slave. So although we are slaves to men outwardly, the one we hold in our heart as our true Master is the Lord. This is not a sanction of being rebellious to our masters but a reference to our heart. The context makes it clear that it is not saying that slaves should seek to be free or to rebel against their masters.
All believers are brothers.
Yes, there are apostles, prophets, evangelists, etc. But we just don't need to address them honorifically. Note that the critic is relying on a particular interpretation of Acts 13, as it doesn't clearly say there is no unpardonable sin. It merely says that those who believe are justified. Now, Jesus' teaching may be descriptive in essense - those who blaspheme the Holy Spirit are those who never believe.