Hi,
These can be used for doing so.
CKey.ChangeNBit(c4d.NBIT_CKEY_AUTO, False)
CKey.ChangeNBit(c4d.NBIT_CKEY_REMOVEOVERSHOOT, False)
By the way, if you want to get correct value of tangents, CCurve.GetTangents() should work correctly, instead of using CKey.GetTimeLeft, etc.