Very cool project by the way! You don't have to do several process lists, you can easily have several 'Is Within Box' nodes within the one 'Build List'. All you have to do is a bit of nesting of the logic.
Here you have three different sources for the 'box'. If the first 'Select Input' is true, cyan will be passed along as the false color for the second box. If the second 'Select Input' is false, cyan will pass to the third box as the false color. If the third 'Select Input' is also false, it will then pass along cyan as the final color. If all are false, blue will be passed along, and if any of the later ones in the tree are true, it will settle at the latest 'true' color. This way, you only need to convert the one color-list to Art-Net. I think the color list would be the simplest option to deal with by far, especially for LED strips.
What are you planning to make? Cymatics and Oscilloscopes are perhaps better placed in a shader of some sorts. I at least haven't found a satisfactory way to emulate that in Vuo (that doesn't necessarily mean it doesn't exist). I see from the way you pose the question that you might attack the problem from the wrong angle though. As audio input is a one dimensional value from 0 - 1 it doesn't lend itself well to xy(z) manipulation in its raw form. What it does well however, is scaling. So if you start by making a circle/sphere, then you can apply the audio values as scaling for those points, either by adding/mulitplying lists, or by scaling the points that go into them.
I'm attaching two compositions I whipped up quickly to demonstrate two ways of doing it, although there are a lot more. I suspect that if you go into 3D space with them, the z-axis (provided the object has origin at 0,0,0) will be the best one to scale.
You have to think about the process/build list nodes as a self-containing loop. This means that the hold nodes needs to be triggered from the loop itself in order to avoid flicker/unintended behaviour. So in order to get it to work, you would have to trigger the orange hold nodes from the orange process list "process item" port. However, I don't think that the process list is the best suited tool to do the task if I understand the build correctly.
If you look at the picture, the Xs mark where signals enter or leave a loop without a hold node, which will make things go bonkers.
As far as I understand it, the process list is intended to use to change items, i.e. if you input a 3D point list, you should expect a changed 3D list output. So when you then want to create a list of color items based on the 3D point list, the build list is better suited. This way you can pull the info from the original list, and create a new color list based on the attributes you set. To make it react to both set values, and an animated value, you can use the logic nodes && (AND) or || (OR) to merge which points it should react to (although at increased resource usage).
I don't think this has much to do with color spaces, rather color selection. The color in photoshop have a K(N) value of 0, which when using CMYK in VUO will render a fully saturated and bright color. It seems like Photoshop derives the grey value for the top 3rd of the spectrum from the amount of CMY, then starts exponentially adding K(N) from that point until reaching K(N) = 80 to 90 at the bottom (totally black). You can either adjust the black level manually (which probably won't produce the same color):
Or use a Make Hex Color node to get the desired result: