You might want to try the following patch. Actually I think you are the person with the perfect case scenario to do so and provide the MG team with useful feedback.
http://teamcity.monogame.net/viewLog.html?buildId=53654&buildTypeId=MonoGame_PackagingWindows&tab=artifacts
Also, ContentManager is not thread safe. If all threads use the same instance you have to make a custom one by overriding a couple of methods and use thread safe datastructures (from System.Collection.Concurrent) and some locks here and there. Otherwise it will fail every now and then with unexpected results.