So when the wave reflects off of the metal plate it loses some energy. This means that the amplitude of the wave returning is lower. So the waves dont fully cancel eachother out. Its like if the amplitude of the first wave is 2m and then the reflecting wave is 1.5m there would still cancel out but not fully as there is still an amplitude of 0.5m left. I cant remember why there is energy loss but usually to do with the distance travelled so if i shouted from 100m away and then from 10m away will nothing blocking it the 100m would be a lot quieter. Again it will be due to the extra distance of reflecting. Dunno about part 2
Hope that helped in some way.