Demand Response Plan - who creates the plan?

Could be any of them or all of the above. Most commonly I think it would be the mechanical engineer.

I agree with Marcus, could be any/all of the entities you mentioned. 

We have kicked this around the office over the years and it seems like the consensus is that the modeler is best to identify places where the load could be shed since the modeler has the best understanding of breakout of energy end use by system. The modeler provides the list of the respective "high' end uses (e.g. interior lighting, space cooling, etc) to the design team. The design team creates sequences to shed the load. The commissioning authority prepares functional scripts with the load shedding sequence included. Contractors demonstrate the load shed sequence while the commissioning authority witnesses. 

As Marcus mentioned, the mechanical engineer is probably the best fit to be the designated author of the plan since they are ones that author the Demand Response control sequence. 

Not surprising we found this to be much more successful when Demand Response (or Grid Harmonization) is discussed during concept phase as opposed to late design and/or preparation of construction set.