Your idea is quite interesting, although I don't know if it is possible. I don't know if devices now a day can sense the sensitivity of the touch.
But I think that it's an awesome idea to give a try using the speed of the touch. So here we go.
You can use a variable with initial value of 0, and every tick that the player is touching the screen you add some value to this var and then as soon as the player releases the screen you stop adding values.
Try this, set some text objects for you to be able to check the speed of every touch, give it some tries, make some tests and you will find out the speed of the touch sooner or later.
If the speed can be negative? Yes it can, when speed is negative it means the object is decelerating. But since you are measuring the amount of time that the user spends with his finger on the screen I don't think deceleration rules apply here.
And yes you can compare more than two speeds, but you will need a different variable for each stored speed you want to compare.