It's really, really important to read the FAQ as a set of guidelines that presume the reader uses common sense when reading it. If you try to read any line in the FAQ as something that must apply 100% to every single question in the same way (and if it doesn't, it's invalid), you're going to have a bad time.
Break down the line into its component parts:
You should only ask practical...
Stack Exchange is for questions that have some utility and are grounded in reality, not for dreaming up scenarios and counterfactuals. Take your first example, "Where is the impossible space in Portal 2?" is practical, "What would happen if Portal 2 had impossible spaces?" is not.
Which leads into
A question that's answerable is one that can be verified in principle. "Where is the impossible space in Portal 2?" can be answered. "Why do video game framerates need to be so much higher than TV and cinema framerates?" can be answered. Heck, even "What's Gaben thinking of right now?" can be answered. They can be verified, with varying levels of difficulty, by testing or asking people in the know.
A question that isn't answerable is one that comes down to personal preference, or is rhetorical, or is based on a worldview or reality structure that's not compatible with the one we live in. These are not answerable questions:
- Who's better: Mario or Luigi? (personal preference)
- Team Fortress 2 sucks, am I right? (rhetorical)
- Why did Gaben make it so it takes 25 hours to unlock all the options in the Paradigm system of Castlevania: Symphony of the Night? (what is this I don't even confusion between three different things)
...that you actually face.
This is what I refer to as the "who cares?" clause. A question can be practical and answerable, yet nevertheless is either uninteresting or so contrived nobody would ever be in a situation where the answer actually matters. A good example are the broad comparison questions that lack any focus or context, but they generally have the hallmarks of thought experimenting rather than actually playing the game:
- What are all the differences between Final Fantasy 3 and Sonic the Hedgehog?
- Do I need to beat Super Mario World 3 to understand the plot to Half Life?
- What are all the instances of the number nine in Skyrim?
One thing should be obvious from all parts of the line, though; its aim is to prevent questions that are essentially stuff you'd ask when you don't really have a question you need help with, but want to ask a question anyway.
And that's not to say that we shouldn't sometimes seed the site or to say something silly like self-answering is prohibited by that line, but that a general, common-sense approach to asking questions should be: "am I asking this question because I want someone to explain something to me, or am I asking this question to provoke a discussion or to stump the audience?" If it's the former, you're probably (not unassailably) okay. If it's the latter, you're probably (not unequivocally) in the wrong place.
But I'm not saying anything new: if you keep reading on in that section, the line is explained in more detail:
If your motivation for asking the question is “I would like to participate in a discussion about
______”, then you should not be asking here. However, if your motivation is “I would like others to explain
______ to me”, then you are probably OK.
This is all to say that every example question you mentioned would count as a practical, answerable problem that someone actually faced and is really in no danger of being closed by an application of the line in question.