1. We have recommended a clearance of 10 mm between the guard and the sensor just to make sure that you don't accidentally trigger the guard sensor. There is nothing wrong in reducing the clearance, but make sure that you don't trigger the guard sensor while touching the button sensor.
2. Yes, you can also reduce the guard sensor thickness. In that case you need to manually tune your sensor in a suitable manner.
Instead of reducing the clearance and thickness of guard, can you try to reduce the button size? We recommend a minimum button size of 5 mm .
Hope this helps!