Divide a video into parts
Most video editors support marks, and importantly "in" and "out" marks.  Generally, if you set your in and out marks for each of the 12-18 minute segments you want and render the sequence, what renders out is not the whole hour, but only the frames within the in and out marks.  If you do that 5 or 4 times, you'll get the segments you want.

There is essentially no way to remove white noise; you can duck it (meaning reduce the volume of the white noise) during times when there are no other interesting audio signals, but when there's a voice you want to hear, that voice is going to blend with the white noise and it will be heard.  Background noise is also very difficult to eliminate, which is why it is always best to pay attention to audio up front.