As far as I know, a person can go into a trance and tune stuff out and yes suggestion from a hypnotist can make precise details of a conversation vague. This happens all the time. If you ever went into a trance while in a car talking to someone, or if you ever listen to music or watch movies, trance induction happens all the time.

If you listen to a song, and the song tells you to go on a killing spree, and you go do it, then you can't sue the song writer saying they hypnotized you with their lyrics even if the song could put a person into a mental suggestive state or even a trance. So to answer your question, it's extremely hard or perhaps impossible to know what a person does or doesn't remember. Perhaps a lie detector test?

There are questions too. In hypnosis in order to get that deep into a trance a person has to trust the hypnotist and go along with it. The direct method of hypnosis doesn't work on a person who doesn't go along with it. This is why it's very difficult to know.

In my opinion, based on all I know of hypnosis, it cannot be done without consent. It is possible to give consent enough to go along with it, and then the memory loss part cause you to not even remember clearly. In that case even if there was somehow consent it seems very stupid for any hypnotist to then say to the person being hypnotized that you should not remember it.

Wouldn't they want them to remember it so as to be sure they have continued giving consent?