Most people can tell how far apart their fingers are via proprioception. It may take a little while to know where the boundaries are, but it's easy to determine

Most people can tell how far apart their fingers are via proprioception. It may take a little while to know where the boundaries are, but it's easy to determine if your fingers are close or far. Feeling how many fingers are touching the screen is also easy with the normal sense of touch. Add in a little hysteresis to handle bumps (you could even integrate info from the accelerometer) and I bet that even bumpy conditions will be significantly improved with this system.