Seems to me that this is being overthought, if'n you don't mind my saying so 318.
I do however agree with everything I have seen so far about the rubber versus rubber with steel core gasket and fluids.
As far as mixing Dexron with Type F, I see no reason at all to do this on purpose, but also know it doesn't matter one little bit if they do get mixed.
Really, what I do is drop the pan and filter and then fire the car up and run it through the gears real quick at an idle and shut it off.
Then put it back together and fill it with fluid.
As far as the delay in reverse, a drained back converter can do that (like Tracy mentioned) and it very well could be just a semi normal thing.
I would say if you go out in the morning and start it up and it has a 2 second delay it's going to be fairly normal.
I'd worry if I had to rev it a little to get it to engage when first started cold.
This would indicate hardening rubber seals in the clutchpacks and rear servo that applies the rear band for reverse
All this said and discussed though, I think you should just drop the fluid and filter and let it drip for a half hour while you clean up your pan and bolts, then put it back together and run it.
