If a learner skips through part of the video, will they get a completion, or not? And how would presence verification figure into that?

If any portion of the video is skipped, the eLearning will be reported as incomplete.

Presence verification prompts users to periodically acknowledge that they are watching the video by pausing the player until they interact with the verification prompt. This ensures that users cannot launch a video and get a "completion" while they are doing something else.  You can set this feature in the edit page of the video, under the compliance menu.