For dealing with the "big thumb moves the touch point downwards" you will want to increase maxDeviation.
Please play around with those properties and tell me if there's a configuration that works well for you. Ie., one that doesn't reject seemingly correct edge-drags but that does reject taps, vertical drags and overly slow drags.
@Pat
You can tweak the gesture recognition parameters yourself. On device, edit the following file:
/usr/share/ unity8/ Launcher/ Launcher. qml
You want lines 192 to 195:
minSpeed: units.gu(5)
For dealing with the "big thumb moves the touch point downwards" you will want to increase maxDeviation.
Please play around with those properties and tell me if there's a configuration that works well for you. Ie., one that doesn't reject seemingly correct edge-drags but that does reject taps, vertical drags and overly slow drags.