Is there an ethical obligation to make author order reflect conventions in your field?
I don't think you're overthinking this. If your authorship decisions are made with the intention to mislead others about the respective roles and contributions of the various authors, then yes, I'd say that is pretty clearly unethical. For instance, your first example can be reasonably interpreted as a mild form of gift authorship.

It's also important to remember that authorship of papers can have real consequences to people's careers. An example that comes to mind is the Alpher-Bethe-Gamow paper, a famous paper in cosmology that provided one of the early theoretical analyses supporting the big bang theory. The work that led to the paper was part of Ralph Alpher's dissertation work under George Gamow. Gamow, who was already a well-known physicist at the time, had the idea to add the name of his friend Hans Bethe (an even more famous physicist) as a coauthor purely as a joke to make the list of authors sound like "alpha-beta-gamma". This didn't work out so well for his student Alpher. The Wikipedia article I linked to above says:


  Alpher, at the time only a graduate student, was generally dismayed by
  the inclusion of Bethe's name on this paper. He felt that the
  inclusion of another eminent physicist would overshadow his personal
  contribution to this work and prevent him from receiving proper
  recognition for such an important discovery. He expressed resentment
  over Gamow's whimsy as late as 1999.


Simon Singh (in whose wonderful book "Big Bang: The Origin of the Universe" I originally read this story), also writes in this article:


  Gamow and Bethe were then both famous names in the world of physics, so scientists assumed that it was they who had done the bulk of the work, which meant that young Alpher was ignored. In the decades ahead, the formation of helium in the wake of the Big Bang would become one of the key pieces of evidence to support the Big Bang hypothesis, but few would remember Alpher's contribution.


To summarize, since the conventions surrounding author order vary across disciplines and are inherently somewhat vague, someone who wants to manipulate author order in one of their papers to give a small boost to someone's career will probably have enough plausible deniability to get away with it without anyone making a fuss. But it shouldn't be done without a very good reason, would be unethical if done out of a dishonest motivation, and can be potentially harmful even if done for innocent or whimsical reasons.