But User:Mass_Driver just admitted, in a place where I can read it, to having lied to me (by saying that the past treatment was merely "playing along", putting up a false pretense that User:Mass_Driver accepts that I am a Clippy). That's hard to reconcile your asserted standards about when humans find it acceptable to lie.

As JoshuaZ said, the rules are complicated. And, since the rules are not designed to handle human/Clippy interaction, you're likely to encounter a significant number of special cases if you take our interactions with you as your main dataset. It may be more useful for you to consider only human/human interaction when figuring out what our social rules are.

In most situations, lying in the way that Mass_Driver did would be a signal that e considers the lied-to party to be of extremely low social status, and that e thinks that other members of the group agree with that assessment and will not reprimand em for communicating that. Such situations are very rare, in normal circumstances, essentially only occurring between a member of the social group that's present (Mass_Driver takes this role) and someone who wants to be or believes they are a member of the social group but is not actually accepted by that group. Behaving in that way toward a fellow group member is generally considered unacceptable, even if that group member is low-ranking. (Even behaving that way toward a non-group-member is often considered questionable, but this varies from group to group.)

In this situation, it's more likely that Mass_Driver actually believes that you are being portrayed by a human, and that that human will consider their social status to be lowered only slightly, or not at all, by the exchange. In this scenario, since Mass_Driver believes that you are not actually real, your social status is not significant to em. One person not believing in the existence of another is relatively new to humans, and our social rules are not set up to handle it. In this instance, your existence is unlikely enough that others are not willing to defend it, and Mass_Driver was allowed to switch to the rules governing discussions of fictional characters, which allow those characters to be spoken about as if they are not present and will never have the opportunity to know what is said about them.

I believe you are wrong about the badness of my lie, and others will disagree with you; and that User:twentythree would have felt more welcome to the site if others hadn't told User:twentythree that my claim about Harry Potter: MoR is false, even if User:twentythree later found out it was false at the time, since User:twentythree would recognize that it was an attempt to make User:twentythree feel welcome.

This varies from group to group and from greeted-individual to greeted-individual. This group has stronger-than usual norms against falsehood, and wants to encourage people who are similarly adverse to falsehood to join the group. In other groups, that kind of lie may be considered acceptable (though it's generally better to lie in a way that's not so easily discovered (or, for preference, not lie at all if there's a way of making your point that doesn't require one), even in groups where that general class of lies is accepted, to reduce the risk of offending individuals who are adverse to being lied to), but in this situation, I definitely agree that that class of lies is not acceptable.