See above. The law is not algorithmically autistic in enforcement. They do not have to charge you for a crime. They can also charge you for something that is barely supported by facts.
In this case, it really wouldn't be reasonable to charge all three of them with joint possession. Unfortunately for Kayla, as Nick's spouse, she's presumed to own whatever is in their property. In this case, even though the owner is apparently a trust, they have full control over and the exclusive rights to the use of the property.
Simply being on someone's property and using drugs there doesn't mean you own every drug they have. If you do a line or two with Tony Montana, you don't own the ten kilos he has in his safe. In this case, the only thing they really have April on is the cocaine directly associated with her own property, and imo even that case is somewhat weak. The argument would be that there is a very strong inference that cocaine on an intimate piece of property, her cards, that ordinarily another person would not have access to, indicates beyond a reasonable doubt that she exerted physical control and, therefore, possession of the cocaine in question.
Technically, if you went by the strict definition of the law, simply consuming drugs would almost always require that you "possessed" them at some point, since you voluntarily exerted physical control, i.e. "dominion" over them at least for the period of time it took to consume them. In practice, this is almost never pursued.
April's case is in the gray territory in between the two. I could see her claiming, possibly even successfully, that Nick asked to borrow the cards and actually did the cutting and that all she did was do a couple lines. That wouldn't help Nick much but it could be reasonable doubt depending on how credible she was about it. Or her lawyer could keep her off the stand and simply argue that the prosecution hadn't eliminated reasonable doubt that it could have been someone else's cocaine even though it was on her cards.
I don't think it's a hopeless case for the prosecution, but they certainly don't have her dead to rights, unlike Nick and Kayla.