As far as the CYA question. Even though it's small enough to pass through the sock. It is still not quite small enough to interact with the test reagent. At least that's the way I understand it. I'm sure there are a few folks that have a more technical answer, but mine works for me...
Cyanuric acid has a fairly low solubility in water, ~0.2% at room temperature. As a molecule, it is a ring structure (aromatic) and, as pure CYA, is not very polar. The log of the first acid dissociation constant (pKa, or equivalent to the pH at which half the molecules lose their their first hydrogen ion) is ~ 6.88 which makes CYA a weak acid. Now a lot of this changes in the presences of chlorine, but those few simpler facts illustrate why CYA is not particularly good at dissolving in pool water. Also, being a heavier, non-polar molecule, it probably has a low diffusivity relative to other chemicals that we add to our pools.
Here's a simpler way to look at it and an experiment you can do at home - take two tall glasses of still, room temperature tap water. Add a teaspoon of salt to one glass and a teaspoon of sugar to the other glass. You'll notice right away that the salt appears to cloud the water and you can watch it almost dissolve right before your eyes. The sugar, on the other hand, sinks to the bottom and doesn't change a whole lot (in the right lighting, you can see the index of refraction difference between the saturated sugar water at the bottom of the glass and the tap water at the top; you can almost see boundary layer between the two). Let them sit still for about an hour or so. Then, using a straw and your finger, pipette some water off the top of each glass and at varying depths. You'll find that the salt has dissolved and diffused much more extensively in it's glass than the sugar has. CYA is a lot like the sugar in the bottom of the glass - it is slow to dissolve and requires a lot of mixing to make sure it is properly distributed throughout the water volume. But note that, once the compounds are dissolved and mixed, they remain completely mixed and do not segregate or settle out.
This is why it is important to get granular CYA in a part of the pool that has a high flow rate of water - you want lots of fresh water around so that the local concentration is always below 0.2% and you want good mixing to occur. The best place to find those two conditions are in front of a return or in the skimmer basket. In front of the return is a better choice as there is a good, high flow rate of water and the sock of CYA is sitting in a giant water volume, i.e., the pool. The skimmer basket is second best because while it has a very high flow rate of water, the volume inside the skimmer is very low so you could come closer to the saturation point of the CYA if your pool pump is on a low speed or turned off. As well, CYA can lower the pH of water to ~4.5 which, in the larger volume of the pool, makes little difference as opposed to the skimmer where the water volume is much smaller and pH that low could damage plastic or metal parts. Finally, as small undissolved particles of CYA make it through the mesh of the sock, they can get sucked into the filter if the sock is in the skimmer. Inside the filter, the linear flow rate of water is a lot slower than in the pipes and so the CYA can take longer to dissolve or get accidentally backwashed out if hasn't thoroughly dissolved. Small particles of CYA getting through the sock and blown into the pool water volume will eventually dissolve.