I don’t have any expertise with which to answer your question definitively, but I wanted to chime in to say that my first thought was exactly this: “hands that are gripped together are unable to present a threat to you, so it is a signal of voluntary vulnerability.”

And rather than vulnerability, it might be more accurate to say that it represents submission, which would tie in with your second question, so it’s not so much that one is signaling that one is not a threat to the god(s), but that one submits.

And in that context, it’s likely noteworthy that the most common example of clasped hands oitside of prayer is when one is earnestly begging something of someone else, and especially a favor or a certain inconvenience.