How are individual mutual capacitance signal read out from a touch screen?
Your understanding is not correct. There is always capacitance between the TX and RX, regardless of whether there is a touch. When there is a touch, the capacitance actually decreases. 

To sense a touch, the capacitance at each intersection is measured. Loosely speaking, the measurement is done by sequentially applying a voltage on the TX lines and checking the voltage on the RX lines. A larger capacitance will charge and discharge more slowly.