Q1. I've been once.
Q2. A mix of XC trails, along with the quarry features, will appeal to a wider variety of riders.
Q3. More mtb trails in an area will bring more people in. The more trails that there are the longer they will stay or they'll return more often.
Q4. Not really.
Q5. Toilets are important for handling bigger numbers of people. Showers would be nice and could be included with the toilet area, as at other trail centres. A bike shop would be good as things break, are forgotten etc. A cafe is a good idea, as it's a meeting place / focal center and will help bring in riders of all types.
TonyL - Good luck and thanks for putting the effort into this.