Ok this one was a tricky one but I think I figured it out. You actually have 2 issues happening.
The first is captured in UIMessageConvertSequenceContext.png attached. For some odd reason you need to convert the sequence context from a variant to a reference before passing it to your event. Also, I highly recommend that you move the property node for doing this inside of the case structure. You never know when another UIMessage will send ActiveX data that is not a sequence context and then you'll run into worse issues.
The second one is that you need to close the sequence context inside of your event (CloseSequenceContext.png attached).
Also, my advice is that what you are doing seems like it could be a huge cause for worse headaches down the road. I would just handle the issue inside of your callback VI and not try to pass the sequence context back to your main VI. The reason I recommend this is that once your callback VI completes execution it returns back to TestStand. TestStand will then continue on and could be done with that sequence context by the time your UserEvent starts to handle it. Therefore you could be dealing with a destroyed sequence context in your user event. Basically you are opening up yourself to race conditions.
Hope this helps,
