Getting the changeling guards to chase him so the rest of the rescue party could sneak through was definitely trickery. He spent much of "The Break Up Break Down" trying to manipulate Spike and Big Mac into playing O&O with him and then secretly arranged for Sugar Belle's cart to break so Big Mac could help her. "Discordant Harmony" is, to reiterate, the one exception where he's specifically trying to not be himself. His means of goading Twilight into chasing after her friends in the Everfree was certainly manipulative, and you can't stop me from also looking askance at watching Twilight suffer through the past trying to figure out what was going on rather than throwing her a bone since he knew exactly what was up. Some of that was for specifically good ends, but as we've both already agreed, that's not a requirement.
So let's consider what he's done recently to see if "A Matter of Principals" is in line with it:
- "The Best Gift Ever"" He causes a winterzilla attack that threatens to ruin the Hearth's Warming gathering. He claims this was to give Fluttershy a chance to be the hero; it is difficult to judge whether this was an actual motivation or a post facto justification based on how things worked out. However, it is likely he was (additionally or entirely) motivated by the slight of not being invited to said gathering, as evidenced by how he passive-aggressively brings his lack of invitation up to guilt Dash. (One will note this in particular as an example of him not clearly expressing himself and instead resorting to passive-aggressive revenge and/or guilt tactics.)
- "The Break Up Break Down": For an episode about bad communication, this one provides surprisingly little evidence either way about Discord's communication skills; he doesn't have that much to communicate. He doesn't seem all that sympathetic to Big Mac's situation for most of the episode, instead giving somewhat questionable advice more aimed at distracting Big Mac from his pain than dealing with the actual problem, but in the end he does help Big Mac and Sugar Belle reunite.
- "Celestial Advice": He engineers a Twilight Sparkle freak-out because it's funny.
"To Where and Back Again" - He objects strongly to Fluttershy's kidnapping, and takes it kind of personally that Chrysalis's throne prevents him from doing so alone. He spends a lot of time bickering with the rest of the ragtag band, although he only once lets it rise to the level of being actively counterproductive.
- "Dungeons & Discords": He tries to stop Fluttershy from leaving Ponyville, although more in a for-show way than in an I-don't-think-you'll-make-me-turn-the-train-route-back way. At the guys' night, it is not his fault that nopony gives his a proper description of how O&O works, but it is his fault that his response to the resulting frustration is to try to lash out in-game, then to lash out out-of-game, then to lash out by putting Spike and Big Mac into the game. After they get him to stop, he delivers a passive-aggressive request to be given a second chance in the form of a guilt-laden farewell, though he does ultimately give an actual apology for his behavior.
So I stand by my claim that he is immature and passive-aggressive.