How to prevent video dragging until after completion?

I think it's best to use the video controls.

To prevent the user from dragging to the end, cover the controls with a filled shape. The solid fill could either have no transparency, partially transparency, or 100% transparency, depending on how you want it to look and how it impacts the bottom of the video.

To allow the user to drag the video control after they've watched the video, add a trigger that changes the state of the shape to Hidden when the media (aka the video) completes.