You'll get stacks of opinions but I think in reality nobody knows.
You'll find apparent answers if you google the subject but when dig down you'll no doubt find the piece you are reading ends with something along the lines of "that is why we recommended...." i.e. it has come from an "in" or "out" biased source.
AFAIK the only known is if there's a "out" vote there will then follow approximately 2 years of negotiations on how the UK exits the EU. That will determine which treaties, agreements and administrative arrangements will still apply, which EU associations and organisations we are still members of and in what sort of membership capacity (e.g. voting or non-voting). At some point in that process, if it happens, we'll know the definitive answers to your questions.