Actually, "engine braking" as you potentially describe is a better and more efficient way to charge the IMA battery. With only 13kW of braking power, more often than not, one exceeds the regenerative threshold and engages the binders rather than capturing the kinetic energy as chemical.
I've owned two of these. If you're seeing forced recharges, it's likely not you, especially if they start at 2 bars. It's likely recalibrations. With a healthy battery, best results are obtained exactly as you describe UNLESS you're doing it in neutral. If you're coasting in neutral, stop. Just back off the throttle as needed and use "engine braking" as much as possible. I quoted that because normal engine braking is actually regenerative braking.
If you are not using neutral, it's the sh!t battery in the '09. They are horrific turds.
There is no way to disable assist. Stop trying.
