According to commentators the pardon was an abuse of power and thus a potentially impeachable offense; a manifestation of Trump’s contempt for the Constitution; a less than propitious omen for Robert Mueller; and a symbol of an imperial approach to the presidency, one which sees its powers as existing to protect its occupant and his allies.
I agree with your conclusion that part of the reason for Trump’s pardon is that he and Arpaio are simply kindred souls when it comes to authoritarian means of crime control. However in your list of reasons above, I think you miss one that was also on target.
This was also a “Racist Pardons Racist.” Trump pardoning a convicted racist is a calculated move. It is a signal to his white supremacist base that he is on their side after all. He panders to that base because they support him, and this pardon is part of that.
This pardon was all about Trump communicating to his racist base that, while he had to talk bad about them for appearances, he is still with them in his heart.