# even if you ... were the signet on my right hand
Yahweh speaks to Jehoiachin as if it were possible for him to be the ring on Yahweh's hand.
# the signet on my right hand
"the signet ring on my right hand that represented my authority to be king". A signet ring was used by a king to place his seal of authority on documents. It represents authority to rule.
# I would tear you off
"I would tear you off from my hand" or "I would quickly pull you off of my hand"