It looks like I was wrong: qtubuntu cannot set those as they (QML_FLICK_DEFAULTMAXVELOCITY, QML_FLICK_DEFAULTDECELERATION) are defined in qquickflickablebehavior_p.h that says: "Platform specific flickable mechanics are defined either here, or in mkspec files."
It looks like I was wrong: qtubuntu cannot set those as they (QML_FLICK_ DEFAULTMAXVELOC ITY, QML_FLICK_ DEFAULTDECELERA TION) are defined in qquickflickable behavior_ p.h that says: "Platform specific flickable mechanics are defined either here, or in mkspec files."