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It may surprise you that this is the best way:

I couldn't find a method for "Convert Instance VI to Standard VI" as you can from the right-click menu but it looks like if you get the VI reference from the subVI you get a reference to the instance VI so you could potentially just copy the block diagram and put it into new VI (reconnecting the terminals might take some work). I'm not sure what the best way would be to figure out whether the subVI is a VIM. If you grab the subVI reference and try to pull the VI Path you just get <not a path> and the VI name returns some crazy UID.

This turned out to be surprisingly easy to fix. The fix will be included with LV 2018 SP1. The problem is that there are a very few nodes in LV that reach downstream to get their types when they think it is safe to do so as a way of giving better default types for terminals. But when the downstream terminal is something polymorphic, that has to be limited or disallowed. I never knew about that feature of LV (though I've unconsciously taken advantage of it many times, it turns out). I just needed to add a downstream malleable VI to the disallowed list so that the malleable is always driven by upstream instead of driving upstream.