There was some discussion a while ago about frame by frame, but there are no mechanisms in the code that would support it, even if the codecs allowed it. I don't know, but if it were going to be part of MP it wouldn't be until MP 2 or later.
There is currently no multi select capability. :sorry:
You could setup an external player such as MPC-HC which has frame-by-frame support, it's not ideal, but at least it will give you what you want until MediaPortal 2 is available.
Setup Follw.it and/or Trakt and use their fast website method to tag movies as watched. Not sure if Follw.it allows an easy single page overview to quickly tag movies as watched, but Trakt does. Then the plugin integration with MovingPictures will ensure that the website tagging will sync up with your local library.
Another local way would be to use SQL queries direct to mass tag, but this is very complex and technical and not for the faint of heart.