Custom interactive Google map project. PHP / msql database

Our website [login to view URL] is a global community for travel enthusiasts. It is built on the Dolphin social networking platform which uses PHP and mysql database. Our Wiki map is the centre of our website and is a facility for our members to add places to the Google maps. Our problem is that the places are added by address alone which means that in many parts of the world it is impossible to place markers anywhere close to the required position. We require that a member who finds that the marker is mis-placed using address should be able to go to their edit area where they will find their map and be able to manually position the marker by either clicking on the map or by drag and drop. The new position needs to save to the database. It is important that only the person who adds the marker in the first place should be able to manually place it other than site admin. It seems that if the map is only available in their edit section this requirement will have been met. Further to that we would like the main locations map to also show the sub category markers (accommodation, nightlife, attractions etc) on the main location map. Ideally these markers would be different colours or designs and there should be the facility to switch them on or off to more easily view the content that the visitor is interested in. Please advise whether our desired modification is possible and, of course, at what sort of investment and over what timescale. We suggest you look at Chiang Mai as this is the city we have chosen to develop as a showcase for the rest of the site. Unfortunately this is where we immediately ran into the marker positioning issues mentioned above as the Google map contains only moderate detail. We would like to develop the existing application and hope that we can keep the location data already added if possible.