Go for it boyz'n'girlz, some good (and fuddled) stuff - although anyone loosing sleep over this lot needs to get out more. For what its worth, here's my (scrambled) ideas...
Believe it or not, the lil' teaser you've got here is (by physical def'n) similar to the issue of windshear on finals which is itself an 'inertia' prob. Bear with me: in a big jet I increment my Vat due a loss in ias I expect due to changing hwc as I descend on the profile close to terra firma. Say on app at 150kias I flop thru an inversion and I go instantly from 50kt hwc to zip. My lumbering tin tube full of in-flight meals and fatties has the inertia (mass) thing so for a few moments has g/s of 100kias with zip hwc. This manifests itself as a redn of ias for a very short period only because the ac is now descending due ma nature and subject to the thrust from the motors (providing a force due system no longer being in equilibrium). All these accelr'ns work to put the ac back to a steady state of equilibrium at the original ias wot was (auto)trimmed prior to the disturbance. Without any messing from me (coz i'm asleep mostly) the ac has a 'dip' in the desc prof. In a little plane the physics are the same but the effects experienced differently due to difference in mass.
Where woz i? O yeah, downwind. Say my wee cessna were able to 'instantly' change direction 90 degs to the downwind bit. For the same reasons inertia/mass prevails and it has a momentary redn in ias before equilibrium is again the status quo.
The rub??? THIS IS ALL TOSH AND RED HERRING. The flight dynamics of ac have far greater effect on your ias than all this turning around in rivers stuff. Fact is that incr in LDD much outweighs circling boat in the decreasing ias stakes. Wot about that moth ridden pitot being blanketed by the door I forgot to shut and granny is dangling out in the flow? Wot about the fact that my clapped out cessna hasnt had the instruments calibrated/leak checked since Wonky Dan the Instrument Man threw them in under a budget? Ever seen an asi needle dancing away in turbulence (vertical gusts) - how about that - downwind is well within the mechanical turb layer?
In short, an amusing topic of conversation in the tea bar but we're in danger of loosing the wood for the trees. Me big dumb-bo so apply basic/sound flying techniques taught me by much wiser man so that don't fall out of sky when turning downwind.
Enjoy!