I just spent more than a day trying to fix an issue calling a model from a .net addin. Tech Support eventually walked through it with me. Ended up being the tool/model name was not as it seems. If you create a model in a toolbox and then change the name of the model by right clicking and choosing Rename in the catalog window, the model retains its original name. When calling the model from outside, the name is wrong and gives an error. The model works just fine from catalog or desktop, but not externally. Point is, if you rename a model, do it in the model builder application or in the properties window.