It will always be a continuous flow. Aside from the cup-full that sits idle in the exchanger prior to use, my system will be on-demand. That's why I expect to see an average continuous flow temperature around 160 degrees. I believe my water to water HX is rated for up to ~7ish GPM, which is more than enough to accommodate my DHW needs.
Using a mixing valve seems counterproductive to me, unless I'm confused... With a mixing valve, I would heat my DHW with the HX, then cool it with the mixing valve, then heat it again with the on-demand system? Or, heat it with the HX, then "heat" it again with the on-demand system, then cool it with the mixing valve?
Using an aquastat seems like the best idea... perhaps in conjunction with a 3-way zone valve? Only problem is that I'm not sure how to wire that. The set-up would go something like this: flow to the on-demand system when feed water temp is <X and flow bypasses the on-demand system when >X, but again, no idea how to wire that in.