Lots of great questions.
First, I’ll just say that “context” is the answer to most of your questions, but not only that, but the key to understanding obstacle-to-obstacle. We informally call linking twists ‘chaining’, and there is an art to it to get the balance just right. I believe you are on the right track!
I think the categories are useful, but I would separate the “structure” of a twist from its fictional severity. There is no rule that twists must be balanced against conditions, but the whole game is fiction first, meaning that the complication should arise from the situation.
I would not pull punches on twists, because those are some of the most memorable moments. For example, once I turned a magician’s feet into rocks, thinking that he would want to get rid of this curse, but the player loved it and kept them for the whole campaign. It was very memorable. Not every twist has to rise to this occasion, but the point is twists add a lot to the game so no one should dread them.
The important parts are:
- With a twist, the original task is not automatically accomplished.
- The same test cannot be made twice for the same obstacle.
- Either the original obstacle has been bypassed, or it remains, and the characters must find another way around it, using another ability or skill against a different obstacle.
1. No redo
I would call your no-redo twists “the original avenue is closed” rather than “there is no new obstacle.” The runes are destroyed, but now they need to find some other source of information.
This sometimes segues to what we informally call “twists down the line.” These are deferred twists that you foreshadow in the moment, but the immediate impact is not apparent. For example, “you hear a rumbling back by the entrance.” (The earthquake caused the entrance to collapse). They discover this later.
Above all else, you want to avoid the “you fail and cannot try again” type of closed ending that a lot of other games have, meaning, it is not like you attack and miss and nothing happens. Something new always has to happen or the situation changes.
2. Lost or damaged gear
Gear twists are great because it works with the pressures of the grind. If you have a backpack full of torches, the player can be confident they will not be in the darkness. But all of a sudden, if the whole backpack is lost (because they previously left it behind and now the twist removes it), then all that lost gear means they have to decide to try and get it back or go on without it.
The gear loss might create an immediate problem, such as losing a light source, or a later problem, such as needing to replace a backpack in town.
3. Ongoing non-condition consequences
These are persistent twists or curses. The burning brand and the curse from the altar are good examples. They create an ongoing fictional and mechanical problem, usually with a specified means of removing it. The important thing is that the effect is concrete and consequential rather than an improvised substitute for one of the actual conditions.
4. New obstacle, previous obstacle resolved
Valid and very common. The spear trap example works well this way. This is where you can start to chain twists together. For example, they fall down the pit, they are buried under rocks and need to get out but fail, and in the next attempt to get out the rope breaks, etc.
5. New obstacle, original objective not achieved
Yes, this can work.
The twist rule only requires the introduction of a new challenge. Therefore, the characters do not automatically get the ritual materials after failing the Scavenger test.
The adventure only says that 1d3+2 tomb guardians arrive to investigate the disturbance. It does not specify whether the incense has already been collected. They can’t bypass the original obstacle anymore in the same way, so the players have to figure something else out. The pressure is mounting and the tension increasing.
The same applies to the Chamber of Ablutions. The suggested twist merely introduces the extinguished lights and corpse candles. Therefore, learning the chamber’s purpose is not guaranteed under the twist result. The GM may reveal it, with the corpse candles representing deeper trouble, or may leave the purpose unknown and require the characters to learn it another way.
6. New obstacle, previous obstacle altered
The key here is the “Fun Once” precedent.
After failing a lock-picking Criminal test and dealing with the patrol, the characters cannot attempt the same lock-picking test again. They must bypass the door by another method, such as kicking it down (health test) or blowing it up (sapper test).
There is also a slight variation that gets into the nuance. Sometimes a test is required because of timing and pressure. If you had all the time in the world to pick a lock, most people could do so (after years of eventual attempts). But you can introduce a twist and then once it is clear, just don’t call for another test because the situation has changed and the pressure is gone. Think of this as an interruption. You pick the lock and fail, monsters show up, you deal with those, and now the lock is no longer an obstacle because there is no danger because you’ve cleared the guards.
Combination
As far as combining a new obstacle with damaged gear, this is allowed if it is one coherent twist rather than two unrelated punishments attached to one failure. Things like a collapsed bridge not only close off your exit but move you to another area. This is really the same consequence of the collapsed bridge, but there are multiple, related effects. A strong twist can have several fictional effects while still representing one clear change in the situation. It stands to reason that if you are on a bridge that falls, lots happens. What is not legit though is adding Injured on top of those twists. You can’t mix conditions and twists even though you would think falling off a bridge would hurt. If the injury is the thing, give a condition and move on.
What is missing and worth mentioning is one of the best twists: “you get what you asked, but it’s not what you wanted.” These are deferred twists. For example, you are trying to make it back to camp, fail your Pathfinder, so hold on, you find your way there, but now there are orcs in your camp. There is a new complication because what you really wanted was a safe camp, but now your camp is ruined. The rules say: “Twists can also be played so that the character appears to have succeeded at their task, but in truth they’ve just waded deeper into trouble.”