I'll allow it. Nuptial
Feel free to critique:
Reverse a dad joke = pun = nup
To marry is a clue to the whole word
The last of the first = the last half of initial = -tial. Were I to rewrite, this would certainly be the place to start.
I understood the OP that you can only remove X coins from ONE chest and deduce the contents of all chests by observing removed coins. What's the lowest X?