Question1: yes, the padding performed for each channels of RGB image.
Question2: the shape of img2 is 484x851x3, when you unfold on 0-dim, the shape of intermediate result is 3x851x3x128, it means that on the 0-dim, it can unfold 484 into 3 pieces with patch size of 128 and stride size of 128. So the final result 3x6x3x128x128 means that on the 1-dim it spilt 851 into 6 pieces.

You can fix the final shape into [batch_size, channels, H, W] or what you want.