And that's a really tough question polka, simply because when Harry does it (and how Harry does it), it seems perfectly fine (which seems to back up my whole if it can be done in a non-lawbreaking way it should be, unless there's a reason otherwise). I don't see a lot of consistency in the fiction (which is perfectly fine by me) which is why I often try to fuddle these things out for myself rather than relying solely on the books. Much of what I'm going off of is the attitude of "What makes the most fun?" which is going to be different for everyone, and will even be different among members of the same table.
I think that the event polka is referring to, was an actual use of mind magic by Molly. Harry only used his Sight to deduce what magic was used on Molly's "victims" (I forget their names). At least, that is how I remember it at the moment
I totally agree with you that it is up to the player to be creative and the story is very important, perhaps indeed it comes first. And if the player is actively trying to get as much out of the Laws without breaking them and without bending them too often, that is super, but the laws are there for a reason. (Which you acknowledge, I know
As long as it is appropriate to the character concept and allow at the table, fine, but effort is important.
Example: I play a psychomancer and I which to block someone's movement through my Channeling power. Do I describe this effect in some detail and then roll, or just roll? As a GM I would allow you to just roll in some cases (especially if you're using something like a rote spell/signature move), but it should be established what you do. Do you temporarily cut of his mental control of his legs? That's pretty grey to me. Or do you cause a slight hiccup in his brain that causes him to stumble, as if by chance? Technically and mechanically the same, narrative-wise, not so much.
Can it be fun? Sure, but when you use magic to deter the conscious control of someone's body subconsciously, it might cause some mental 'scarring'. "Why won't my legs move? I now I can move them, I did so just a second ago! I can feel them! WHY WON'T YOU MOVE?!" This is relatively minor, of course, but it can get out of hand.
Imagine that, besides the psychomancer, there is also a wizard (with Spirit-element) in the group, played by a somewhat lazy person narrative-wise. He sees how effective the psychomancer's block was and says, "Yeah, I can do that too! Let me roll and I'll keep the other guy from moving too." Well, let's just say I'd keep my eye on him/her. If it happens more often, I'll simply 'compel' the fact he's copycatting another character's move, but imperfectly and screwed up or something. Then again, I get the feeling I'm a somewhat vindictive GM :p
So, what I'm saying is, if someone puts effort into describing how they want to do something for the sake of the story, instead of for the sake of mechanics, I'd be a happy GM