The poison is always in the dose!
I think you got some previous advice that some oak structure was NOT harmful to the fish in those poster's ponds. It is difficult to know where "some" oaks plus a few more becomes toxic.
Spitball ideas:
Do you have excess watershed (and water) for your pond? If so, you could let it fill to cover the oak branches for a few weeks, and then drain the pond before your next big rain in the forecast. I suspect one or two "soak and drains" before stocking MIGHT remove a significant amount of your tannins.
It is obviously easier to put in all of your structure before the pond fills. However, could you put in all of your deep and/or complex structure now? You could then add more oak tree structure in 1-2 years after the first load of tannins has dissipated. Either shallow structure during a drought or when you deliberately pull down the water level, or deep structure that you just lash together and then weight and sink in place.
As to your other questions, yes, the oak leaves also contain tannins. Definitely build your structures with leafless oak wood.
I believe air-dried deadfalls will have less tannin than fresh-cut, but I also believe the tannins are still preserved below the surface. (I know the oak flooring guys say you can bring up some old tannins if you sand deeply enough. I am definitely NOT an expert on this topic.) However, I believe any deadfall oaks that have been rain saturated and dried several times should have reduced tannins compared to fresh cut trees.
I would certainly use any hemlock trees that are deadfalls, or volunteers that you want to clear. They are conifers (no tannins) and provide good "fluffy" cover in the first few years that your forage fish will use for hiding places.
Good luck on your big "structure" project.
