First of all, sorry for the late reply. Most of us were out of office and this is the reason for the delay.
I investigated the problem a little bit with the assemblies you have provided and it seems to me that the issue is in Reflexil plugin's source itself and the fact that you are trying to manipulate a Silverlight assembly that references a normal .NET assembly. I'm not sure how exactly is your assembly compiled (I'm not familiar with Unity) but Reflexil fails to compile such a combination of assemblies, stating that there is a duplicate reference of mscorlib.
As fixing the issue for this corner case in Reflexil might not be that easy and fast, I would like to suggest you two options:
1. If you own the source code of the assembly, modify it directly there and compile it using Unity.
2. If you don't own the source, extract the code with JustDecompile via the "Create Project" option -- it will generate you a complete Visual Studio project with the code of the assembly. Then modify it there and use it wherever you would like to (for example, Unity).
Thank you for your patience and sorry once again for the delay of this answer.