Thank you for bringing forth those examples! I’ve learned something!
TL;DR — Double Ob penalty does apply! Don’t forget to apply modifiers!
It looks like the circumstances where Stat vs Skill arise in the Fight mechanics are: Charge/Tackle against Throw; Push against Block, Counterstrike or Throw; Lock against Block, Couterstrike or Disarm; and actions played against Run Screaming. This is why Avoid is treated specially.
The letter of the rules here seems clear: When you’re taking an untrained action against a trained opponent, you get double Ob penalties. Period. The spirit implies something like this: When you’re acting on raw ability against a trained defender, you get double Ob penalties.
In light of that, I think I’ve been misreading Unskilled Tests as depending on the existence of an appropriate skill. By my logic, should there be no appropriate skill, it shouldn’t apply. But I see now that there is no such implication; if you do not have any skill that is appropriate, Unskilled Tests applies. Curse my fallacy.
Therefore, I think you are correct that per the rules as written, Evoking against a physical stat imposes a double obstacle penalty.
Above, where I mentioned Speed for positioning tests in my previous comment, I should clarify (for myself at least) that using Evoke in this way shouldn’t be directly possible. The spellcasting would have to be a separate, scripted action with the intent of opposing their positioning test by knocking them down, immobilizing them, etc. and the timing of the tests is somewhat unclear and Sorcery wouldn’t be a replacement for testing Speed to position yourself—suffice to say it’s not a good test case for Evoke even if your table would allow that interaction.
With all of that covered, I wanted to address this part of your question:
As Evoke states that you must use a Versus test between Sorcery and the Skill / Stat affected, would you apply the double obstacle penalty of Sorcery vs. Speed to prevent an enemy of being “frozen” in place? We’re doing that way since the first session and, obviously, there are few NPCs that can resist the effect, rendering them defenseless.
First, if your table accepts completely immobilizing an NPC as a valid intent for a spell (which is acceptable and entirely radical in my opinion), still don’t forget to apply modifiers to the test. Just opposing the target for a single test they would take with Speed, and no other ability, would come with a +1 Ob penalty. Such an ensorcelled victim could still try to run away with Speed anyway, they’d just do so versus your Sorcery in addition to your and your friends’ approach.
To resist them fleeing, defending or fighting back for an entire conflict, that’s probably Speed and martial skill at least, so, +5 Ob. One target +1 Ob, two abilities +3 Ob, the conflict +1 Ob.
To render an NPC entirely immobile so that they cannot take any action whatsoever for entirely long enough to murder them, full stop? +1 Ob for ‘Conflict’ duration, +1 Ob for one target, +9 Ob for “All Abilities”. That’s +11 Ob. A double Ob penalty on some of the tests won’t matter if the sorcerer immolates themselves trying to cast a spell like that.
You’re playing it right! Double Ob penalty applies.
Don’t forget to clearly specify
intent your intent, down to which abilities Evoke is resisting and what physical force affects the target(s), and apply all the relevant modifiers.
Keep having fun!