C# dll "MissingMethodException" - Before first recompile

Hi

I'm using a 3rd party C# dll in my project. It works great most of the time, I can use it without anything throwing errors. But if I have just opened Unity, and click play. It will throw a MissingMethodException when it tries to access some function from the dll. Then, if I just change the name of a folder, make a small edit to a script, or something else which causes scripts to be recompiled, it works perfectly again until I restart Unity. If I try to build before unity has recompiled anything, I get compiler errors saying much the same thing as the MissingMethodException (but if it has recompiled, it works great).

I can solve this by putting the .dll file in the Assets/Plugins folder. However I cannot do that since I am including this in a package which is to be released on the Unity Asset Store.

So my questions are, why does it fail before any recompilation has been done. Shouldn't Unity detect the dll? And is it possible to have a Plugins directory in a package released on the Asset Store (which works), or another way of making sure the dll is compiled before the rest of the code?

I cannot but my whole package inside the Plugins or Standard Assets folder because I also need an Editor folder.

If this cannot be done, I am asking all Unity developers out there to improve on their compilation order system so we can manually specify compilation order for certain folders.

I do have the source code for the dll, but it really seems very ugly to paste that into the project, especially since I will have to edit a lot of defines to make it work properly.

Thanks in advance!