This issue really has nothing to do with garbage collection or setting something to null. From my test of MG's problem, there is an issue with freeing items from memory when there are LODs involved.
If I load a model that has 6 LODs and free the model when i am looking at the main mesh, it will show in the log that the main mesh, the model reference, and all of the LOD meshes are being deleted.
If I load a model that has 6 LODs and free the model when i am looking at LOD2, it will show in the log that only LOD2, the model reference, and LOD meshes 3 thru 6 are being deleted. But the main mesh and the LOD1 mesh are not. And I can verify that if i load the model again as it will only load the items that were deleted.