Plunging or inversions seem to be rather irrelevant - washing is not developing or fixing, where the small amount of chemistry entering the gelatin gets depleted or saturated, and are influenced by surface pH levels. And it needs no even action across time either - as long as the specs are met at the end it does not matter whether the centre washes twice as fast as the perforation margins. The amount of chemistry in the emulsion is tiny compared to the amount of water, and the little bit of hypo and silver eventually dissipates on its own in the water - even if there are local pH effects to start with, after a few water changes concentrations will be down to a level where movement can't distribute it any better.
Ages ago, one German chem vendor (I suppose Agfa) published a water-conserving wash cycle, and the proposed process, with precisely timed (double clearing) fix in good speed fixer, rinse, hypo clearer, rinse and a mere five three-minute stand and replace cycles proved good enough to bring films to archival standard. I have done roughly so (bumping it up to seven cycles as a safeguard for faster, thick emulsion film) ever since, for more than twenty years, with no faded negatives - while some of my earlier negatives, with sloppy 5min fixing in uncontrolled fixer and 15min running water wash, have turned yellow or spotty.
Given that you can get away with very small amounts of water, warming it up to 20°C should be feasible.
Sevo