When Dharamsee had his miracle with the seedless grapes, was anyone else's experience similar? Specifically, what about Tarkhad's grapes?

No, the miracle appears to have been exclusively for Boss Dharamsee. Chapter 35 explicitly contrasts his experience with that of another devotee present, Tarkhad. After the miracle, Dharamsee, in his amazement, asked Tarkhad about the grapes he had received from Baba. Tarkhad replied that his grapes did have seeds. This confirmation, as described in Chapter 35, made Dharamsee "extremely surprised in his heart." The fact that Tarkhad's grapes remained normal highlighted that the miracle was a personal and direct response to Dharamsee's own internal doubts and skepticism.