Video: What are I-frames and why do they affect the size of the video?
You should do some research about GOPs ( Group of Pictures ) to find the definition of I frames.

This will answer the second and fourth question as well.

In generall, the more I frames a GOP has:


the better is the seeking qualitiy. (If the seek lands to a non I frame, it takes ressources to calculate the frame. To avoid this, sometimes seeking is only possible from one I frame to another)
the bigger is the video ( as I frames are saved as full frames )


The GOP has usually no effect on sound, but the codec does.