When the grapes miraculously became seedless for Boss Dharamsee, did this happen for everyone else who received them?

No, the miracle of the seedless grapes appears to have been a personal experience specifically for Boss Dharamsee. The narrative in Chapter 35 makes this clear. After his own surprising experience, Dharamsee asked another devotee sitting nearby, Tarkhad, about the grapes he had received from Baba. Tarkhad replied that his grapes did, in fact, have seeds. This confirmation caused Dharamsee to be "extremely surprised in his heart," reinforcing that the miracle was a direct and personal response from Sai Baba to his own specific doubts and preferences.