1) Incorrect offence location (marking the intersection where he stopped you versus where he observed you) is not a fatal error.
2) Depends on the area where you were stopped. Built up areas (pretty much anywhere in a city) the speed limit is 50 km/hr unless otherwise posted. If it was a rural area you might have an argument.
3) Demerit points are something the MTO applies after your conviction and have nothing to do with police. The officer telling you the wrong amount is irrelevant in Court.
In my experience they're less likely to further reduce a ticket that already has been, but your original reduction was relatively minor and you're not asking for much more.
As for the cop not understanding the point implication, it really means nothing. The earlier post is correct, points are placed after conviction by the MTO and has nothing to do with the police or court.
You best bet is to always select a trial option and at least review the disclosure. There may be something in there you can use to get the ticket thrown out or win at trial. Either way, you can always plead guilty later after reviewing the evidence and realizing you don't stand a chance.....Always at least request the evidence.
You are going to have a record anyway.$10 or $20 up and down doesn't make difference for the insurance purposes .The points are the last thing to worry about.
At least you have a chance if the cop doesn't show up.