Slow-down should surely be helpful...I would think that ideally you want the words-per-minute to approximately correspond to the speeds seen in infant-directed-speech.

I'd generally opt to watch a movie that you've seen many times but don't remember word for word, to prevent translation, so I wouldn't personally watch the subtitles before reading - but not everyone is me so maybe you should test a few people first?

In terms of subtitles, I'd say fewer words, more high quality is better. The most useful thing I think would be explanations of idioms, expressions, and other things that can't be understood literally...as well as explanations of un-common words.

For example, if a movie had the line "You're really gonna hand over the Avatar for a stupid piece of parchment?" then the subtitles would say "parchment - a primitive page made out of animal skin", since you couldn't possibly know that from context. (though I suppose one might argue that this type of information is not very important for a speaker starting out)