1 I can't see a particular ground's marker
On a wide view when grounds are very close together (e.g. Liverpool and Everton) the markers may overlap. Simply select the relevent region to narrow in the view or use the map controls to zoom in further.
2 Is Wembley on the map?
Any grounds not in a specific division such as Wembley or the Millenium Stadium are available via the 'all' grounds option. Either select it from the 'grounds' drop-down box or select the 'all' option from the 'division' radio buttons.
3 Does the map show the start or end of the season?
The map shows the position as at the start of each season.