All good thoughts. My take is, that with 3000 of 5000 the OP really needs an unfair solution. As far as I can tell deNATing is only important for the per-internal/external fairness modes, that will not work for the OP. BUT, I certainly would try with layer_cake and manual setting of the twitch traffic to EF first, as you I have a hunch that might already be good enough ;).
Now if the described priority for the twitch stream is a hard requirement, modding simple.qos might be a better approach.
Unless someone really ambitious sets out to make cake's min/max bandwidth per tin configurable 
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