Hey! The latency that you can get largely depends on your buffer settings and the complexity of audio processing you’re doing. For example, by default, the MODEP image is configured with 128 frames, 2 periods. At 48kHz sampling rate, that would translate to ~5.3ms. To get lower latencies, you can reduce the frame count (less than 2 periods is not recommended), or increase the sampling rate to 96kHz or 192kHz. In general, the formula is (frames x periods) / sampling rate in Hz.
Yes, the Pisound’s output is ‘line level’, so you may hook it up to other devices.
We recommend using MODEP with guitars or if you otherwise want to have an effect unit 
If the screen attaches via the GPIO pins, then likely not. The pins on Pisound are not long enough to stack something on top.
You may use HDMI screens, or ones that connect to FPC connector on Rasperry Pi.