On Exhibit A: Because the player created the aspect, they can use their free tag to compel the NPC to be unable to give orders. The NPC gets a Fate Point, but the player doesn't have to spend any for that first use. Aspects never have an effect on dice rolls or characters' behaviour and actions unless invoked, tagged, or compelled.
Removing the aspect depends on how well the player rolled to put it on the NPC in the first place (you did have him roll for it, right?). If he rolled just equal to the NPC's Discipline roll to defend, then the aspect is removed automatically after the first tag. Essentially in this case it would mean the sand is coughed out quickly. If, however, the player rolled above the NPC's Discipline roll, the aspect is "sticky," meaning it doesn't go away until some action is taken to deal with it.
In this case, that could mean a number of things, from the NPC stopping to cough up the sand, receiving the Heimlich Maneuver, or some other action that would unblock his windpipe. The difficulty to do this is up to you, depending on the circumstances. Out of combat, it typically shouldn't take anything other than time. In combat, for something simple (like throwing water over someone to remove the aspect "On Fire") no roll should be required, but for clearing a physical blockage in someone's throat, that might need a skill roll.
A good rule of thumb is, whenever you do call for a skill roll to remove an aspect, to set the difficulty at the same level as the result on the player's skill roll to place the aspect initially.
Re: blocks, you are correct, the player would have to continually apply the block in order to maintain the effect.
Exhibit B: In this case, the player could have simply treated the throw as a regular Fists attack, rather than using the grapple rules, especially if his goal was to hurt the NPC.
Players should be encouraged to think beyond the mindset that they need secondary beneficial effects to their actions. The only things that happen in a DFRPG game are the things that the players themselves choose to do. So, the player must decide whether they want to inflict stress, place an aspect, or perform a block. How they then describe that is up to them, but can absolutely include description to the effect of throwing someone against a wall, if they choose.
As for the aspect you chose to apply, yes doing so was specifically against the rules, and not exactly in keeping with the paradigm, either. Temporary aspects in combat are only applied to characters through maneuvers and consequences. You don't get to impose an aspect as a bonus to a different action.
An aspect like "Spread-Eagle on the Floor" is kind of poor, because the only way for it to remain in effect is to continually compel the NPC to remain on the ground, unable to get up. It's perfectly rules-legal, but you're players are going to burn a lot of Fate Points and he's going to quickly build up enough that he can buy off the compel and still be sitting pretty on a nice stack of points to spend against them.
If he stands up, the aspect is gone. No difficulty, no dice roll.
If you want to represent a character being at a disadvantage due to a maneuver, you're better off coming up with one that describes a more general condition. Something like "thrown off balance" is good, because it doesn't inherently suggest that a character can't act, but it's still an aspect that can be reasonably tagged, invoked or compelled to create difficulty for the character.
But remember, aspects like that are inflicted through the use of maneuvers only. The only time an attack can inflict an aspect is when the character chooses to take consequences.
If you want a character to be put in a situation where their ability to take action is more seriously impacted, you're better off using a block.
