There are two ways of doing it.
The first is to change the layer's initState property, which determines if it shown or not when the file is opened.
This is the easiest way of doing it, but the problem is that this property is read-only in Reader, so unless your file is only used in Acrobat it's not really feasible.
The second one is to use a script to show/hide the layers based on some logic when the file is opened, using a doc-level script.
The logic will probably be the same as what you're already using for your drop-downs and other fields, you just need to adjust it so that it gets executed when the file is opened, not just when the values of these fields is changed.
I use buttons for things like this instead of layers. I find they're easier to work with, faster, and you don't have to deal with the problem you're having. So rather than show/hide layers, you show/hide read-only buttons that are set up to display the content you want. Set them at the top of the tab order. Using hierarchical field names make control of the fields simple. The layer approach also has less support in non-Adobe PDF viewers. One thing this means is that an initialization script might not get executed (or be fully supported) and the display will be wrong.
The main problem with buttons is they don't have an opacity setting, so you can't use them to overlay items one on top of another, which is usually what people do with layers.
That really depends on how the button icon content is created. Often times a PDF page will have what is effectively an opaque white background, but it doesn't have to. I usually wouldn't have multiple buttons overlayed and set to display at the same time though.
Thank you very much for the help. The document level script was exactly what I needed
Have a nice day everyone.