But given that the pardon involves a wide swath of classified data disclosures, only a minority of which have any obvious "American civil liberties!" tie, how do our hypothetical future leaker determine that a pardon might be appropriate for him, while a hypothetical future spy determines that a pardon would not be granted in their case (especially if they leak something of public import simultaneously)?
A pardon sends a very mixed message here, precisely because Snowden has crafted a very mixed message.
Pardons are purely discretionary acts of Presidents, not applications of legal principles that any reasonable person can expect to be applied in future cases; absent a pattern by multiple Presidents, no sane person would see a past pardon as a reliable precedent—at most it's a source of hope that if a similar political situation is created as a result of the act that there might conceivably be a similar result.
A pardon sends a very mixed message here, precisely because Snowden has crafted a very mixed message.