I'll just go down the list and answer as best I can :)
- It doesn't look like any of your portfolio items have multiple images. Is this incorrect? The next/previous will only work for the individual gallery of the portfolio item.
- To make the videos pop up in the lightbox, you need to add an image, then Apply the "Link manually" option and put the video URL in the input field that appears below that option. This will give you a preview image of your choice and when clicked a pop up video.
- The extra space is needed for the sliding page effect (in case you added more content after it)
It looks like you've turned off the portfolio pagination. I'm guessing you might have been trying to do the options in the main theme instead of on the One Page portfolio dynamic template option.