Thanks for the response. I'm a new GM, so I probably overlooked an obvious bit of text. Where do you see that I should use the passive perception score as the skill DC against Sleight of Hand checks? Since this is in combat, I would think it would be more like a dexterity saving throw or targeting the enemy's AC. What about Deception checks? I see some suggestions that Insight is used as an opposed check (so not passive if my reading is right), but that you'd want to use an Insight (Dexterity) instead of Insight (Wisdom)?
A Dexterity Save and a Sleight of Hand check are, essentially, the same thing. Dex+Proficiency, rolled against a target. Mist and Shade goes in on making it Sleight of Hand because it's about lifting something off someone in a manner that they don't notice, rather than just yanking it out of their grasp.
I notice that some of the maneuvers define the DC, just as Armor Lock
Combat Maneuvers which force saving throws typically use the Maneuver DC of the attacker. So a Fighter using a Disarm basic maneuver makes an enemy roll a saving throw against 8+Proficiency+Dex or Str, whichever is higher.
Your Maneuver DC is defined on page 457 of the Adventurer's Guide.
Every creature has a Maneuver DC, whether they ever use it or not. And everyone has access to the Basic Maneuvers like disarm, grapple, grab on, knockdown, overrun, and shove. Martial Classes (like Fighter and Adept) get additional options for combat maneuvers.
As far as Armor Lock targeting AC: As designers, we get some big leeway to mess around with some fun levers in making things varied and different!
Which makes me think there's a general rule for this, but I can't find anything under the general skills heading, Passive Checks, or the specifics for how to handle this.
Pretty much every skill can be made into a passive by doing the simple 10+Stat+Proficiency method, which makes it super easy to adjudicate things in a way that feels reasonable to both the attacker and defender.
The most commonly used passive skills are Passive Perception, Passive Insight, and Passive Stealth.
Generally speaking, whenever possible, let the players roll the dice. If you have an NPC lying to the players, let the players roll Insight against the NPC's passive Deception. If NPCs are trying to sneak up on the party, call for Perception checks against their passive Stealth.
This is generally because of attacker's advantage. Since you need to equal or exceed the DC, it gives them an extra 5% edge to beat the NPC. Though you can also flip the table, sometimes, and roll NPC checks against the party's passive values to give the bad guys the attacker's advantage.
I really appreciate the help, since I basically don't have any knowledge of 5E or D&D other than the A5E rules and a bit of Baldur's Gate 2 modding back in the day (which used a modified version of the 2nd edition system and pretty much didn't have skill checks other than flat ability score checks when they were rarely used to open up additional dialogue options).
You'll find that people on this forum are always eager to help. And some of us actually write official A5e material!
Also, I really like the Esper as a more fantasy take on a psionic character. It was a perfect fit for how to handle psionics in my group's homebrew setting and consistent with the little bits and pieces of psionics adjacent content we already had (psionics is rare and mostly unknown, but certain species are able to make use of it).
I'm really happy you like it! I think it's some of my best work, conceptually speaking. It really made a splash!