""The difluoroethane in products like "Dust Off", typically used to move dust off film and into the air, is unreactive and will displace air quite nicely.""
I had thought of doing this but hydroflorocarbons never seemed like they qualified as inert. I would try it by I am not a chemist and am concerned about reactions in chemistry I don't completely understand: C41 for example. None of my B&W chems really need this - I do use marbles - dark glass and mix only what will be used in a short time.
As far as presoaking goes - I can't imagine not presoaking 4x5. I think 35mm would probably not care. My first 4x5 tank was square. I wasted a whole box of film learning that square daylight tanks are the wrong answer - I always got uneven development. Even with the Jobo tank, I always presoak off the anti halation layer. I read (Adams) that it takes 20 seconds for the chems to permeate the emulsion. It makes sense to let the chems displace the water evenly as opposed to permeating the emusion at different rates in different spots. I also don't want that unknown chemical disolved layer in my developer. Of course C41 was designed to run that way.