Okay first of all I should nail my colours to the mast. I did not attend the event nor do I own a car. I did however notice that Devonshire Road was very busy.
The organisers of this event could learn a lot of lessons of regular sporting events. I normally attend the Crystal Palace Athletics, which is of a similar scale, and rugby at Twickenham, which is obviously much larger. At both these events there is dedicated parking, and marshalls at railway stations giving out directions and also return timetables of trains. Parking is the key issue, and there is Dulwich and Sydenham Golf course just up the road. Getting agreement to use this is, I would suggest, going to be somewhat problematic.
Food and drink should be controlled in that what you buy in the area, you consume in the area. You could have people on entrance and exits to the park stopping people taking food outside and thus controlling most of the litter problems.
Publicity is very important so that people know exactly what to expect at the venue.
All the above would cost, but if you are going to have an event on this scale then it needs to be done professionally, and you consider charging an entrance fee.
Not to worried about the crime element. I feel its very much luck of the draw in London. For example the front door of the office where I work is literally 30 yards from where there were shots fired at police recently near Waterloo.
Finally on a really controversial note is could be spread across 2 days since it is would be unlikely for people to attend the same event 2 days running. But then again they might