When Mormon uses the introduction "And now…" he is typically moving to another subject. Thus the literary unit that discussed the political organization of Alma's people has ended, and Mormon is moving to a new topic. This new topic is the religious organization of the people.
What Mormon has left unsaid is what type of government the people did have. He specifically argues against kingship, and we must suppose that he was sufficiently persuasive that the people of Alma did not have a king. What did they have?
While we really do not know, we may speculate that Alma's influence over the people was tremendous, he being their introduction to the gospel and the unifying influence over the creation of the people. While Alma may have been disinclined to claim kingship, he would still be the de facto ruler. In this verse we specifically find him as the high priest, which would probably indicate a religio-political position (remembering that the priests of Noah also held combined religious/political posts). Thus Alma continues to be the leader, but not as a king.
What difference is there for Alma in one man ruling the people as a high priest, or one man ruling as a king? The difference is in the connection to the gospel. Remember that Alma's greatest argument against a king was the tendency of kingship to lead people away form God. As a high priest, Alma is aligned with God, and affirms that the principles of the gospel will inform public life. Once again we have underlined the nature of Alma's objection to kingship. It is not the concentration of all power in one man, because it is very likely that Alma holds that same power as the high priest in what is most likely a theocracy. Rather than the locus of power, it is the righteousness of power that makes the difference for Alma.