Youtube – Customizable alarm clock/set off a YouTube video on a timer

Right-click on the video and select Stop downloading. To start it up again click the pause/play button.

Another way is to move the slider all the way to the end or near end of the video timeline. That way only the few remaining seconds left to play will end up streaming down to your computer and when it finishes, that's it. No more buffering or loading.

You can't turn off the annotations in the URL but you can in the embed link by adding iv_load_policy=3 as parameter. Source.

For your second question, other parameters can be found here as well, though note that the scheme is ?firstparam=1&secondparam=42&..., the hash is generally not used.