Iis 7.5 - Error Loading Third Party Dll From Asp Application

the situation is quite complex ( and my english very basic) but let's try to explain:

i'm developing a Asp.net web service calling a method from an external dll. This external dll calls some method from other .net dlls. So we have:

Asp.net WS ----> External.dll ----> other.NEt.Dll(---> other .netdll)

You have to know that the external dll uses a path ( given by an initialize method) to resolve its internal reference. In conclusion i have a web application with an added reference to my External.dll and a fully trusted path ( c:EXTERNAL ) full of all the .net dlls needed by external.dll. Looking around i've found this code to add to the application_START:

Dim path As String = String.Concat(System.Environment.GetEnvironmentVariable("PATH"), ";", "c:EXTERNAL)
System.Environment.SetEnvironmentVariable("PATH", path, EnvironmentVariableTarget.Machine)

this add my c:EXTERNAL to global environment PATH. With this configuration running from the Visual Studio development server i get no error and it all works correctly. When i publish the application on my local IIS server it gives various errors: At first the result is something like:

Failure reading <Myobject> control of <(static)> type
Unable to create <myobject> object (<C:(WRONGPATH)
eeded.net.dll> assembly)

To resolve this i've tried to add the needed .net dlls to the /bin of my application in wwwroot but the result is something like:

Failure reading <MyType> control of <Myobject> type
Error returned by .NET Framework:
System.ArgumentException: Un oggetto di tipo 'ComNet.BaseControl.LoginDisplayLayout' non può essere convertito nel tipo 'ComNet.BaseControl.LoginDisplayLayout'.
in System.RuntimeType.CheckValue(Object value, Binder binder, CultureInfo culture, BindingFlags invokeAttr)
in System.Reflection.MethodBase.CheckArguments(Object[] parameters, Binder binder, BindingFlags invokeAttr, CultureInfo culture, Signature sig)
in System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
in System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
in System.Reflection.RuntimePropertyInfo.SetValue(Object obj, Object value, BindingFlags invokeAttr, Binder binder, Object[] index, CultureInfo culture)
in System.Reflection.RuntimePropertyInfo.SetValue(Object obj, Object value, Object[] index)
in CDotNetType.bSetProperty(CDotNetType* , Object gcrObj, SByte* pszNom, CSLevel* pclPile, Int32 nDimension, Int32* pnTabDimension, STOperationDotNet* pstOperation)

this time it looks like it's loading the same dll but from different location causing conflicts. Now that's all. Its hard for me to explain this dll-hell but basically i would like to replicate whats happening when the application works well in the visual studio development server. I've also read that IIS does not resolve Added PATH without rebooting so i tried to add c:external manually to the PATH and reboot but same errors appears.