I imagine it is open until park closing but not sure. If you care to post your reason for asking someone will probably have an alternative for you if necessary.
Yes anyone can use this entrance, the friendship boats from MGM and the Epcot Resorts take guests to this entrance. As for parking you would have to park at MGM or one of the Epcot resorts (not recommended) and will likely get a 3 hour parking pass at the resorts. FWIW there is really no advantage to entering/parking here its more for the convenience of the guests at the BW, YC/BC and S/D and those coming from MGM via the boat.
TJ