Does imagination and thinking that lead to a sensation of "a throb in the vagina" and the discharge of white or viscous fluid, or sometimes no discharge, constitute masturbation? And does it necessitate Ghusl or only Wudu?

If the discharge that exits is white and emitted with pleasure, followed by a weakening, then it is semen (مني) and necessitates a full ritual bath (ghusl) of the body. It is advised not to indulge in such thoughts and to occupy oneself with what is beneficial to avoid exposure to provocations and satanic whispers.