I fail to see why you would be unable to get your seat fixed under warranty at no expense to yourself at any dealership, unless it has been damaged due to overuse.
It is impossible to recuperate all the energy used to accelerate the vehicle, since a significant portion of that energy dissipates as heat in the motor, the AC inverter, the battery, and even the wiring. Subsequent losses will occur throughout the regeneration process. I am now retired, but I once worked as an electronics engineer. I often assumed that a sophisticated energy conversion system would achieve around 80% efficiency. Therefore, for bidirectional conversion (acceleration followed by regeneration), I would estimate an efficiency of 64%, calculated as 80% of 80%. Therefore, I concur strongly with the 65% you proposed.
In iPedal mode, one does not just remove their foot from the accelerator pedal to prevent excessive braking; instead, one modulates the accelerator position to get the desired speed at the specified point. It requires some practice. In principle, one might equilibrate acceleration and regeneration to effectively coast.