Thanks for writing in!
1.) At the moment, Ubermenu may not work in the header builder. This is something we need to investigate further.
2.) The video format needs to be converted into *.mp4, *.ogv or *.webm. These are the only file types that is compatible in most modern browsers. You may need to increase your upload limit or upload the videos in other 3rd party sites.
To increase upload limit, you might want to check out this video:
3.) Yes you can. Sliders can be assign above the masthead or below the masthead. For more information, please check this out:
Hope this helps.