When Boss Dharamsee's grapes miraculously became seedless, did the same thing happen to the other devotees present, like Tarkhad?

No, the miracle of the seedless grapes was specific to Boss Dharamsee. The source text from Chapter 35 makes a point to clarify this. After Dharamsee's profound experience, he was amazed and his mind was stunned. To confirm what had happened, he asked another devotee sitting nearby, known as Baba Tarkhad, about the grapes he had received from Sai Baba. Chapter 35 explicitly states that when Dharamsee asked him, "How were your grapes?" Tarkhad replied that his grapes did have seeds. This confirmation caused Dharamsee to be "extremely surprised in his heart," reinforcing that the miracle was a personal response to his own specific doubts and mental state.