When I say that the Box of Bog Bryg held an opera, I mean that it in fact held the entire 100-page score to an opera: Atalanta by George Frideric Handel.[1]
Naturally, we all understood — to varying degrees but all at once — that Remy’s last words pertained to this opera: Atalanta’s 15th aria, not Atlanta’s fifteenth area. We consulted the score’s index to find the 15th aria, and, as we should have expected, the vocal part belonged to the character of Irene. Unfortunately, Irene’s notes were written in C clef, but Jeanette actually made short work of them and played the melody without error on the pianoforte.
Not long into the aria, the lantern glowed pink, and we knew that we had found our key. We directed the lantern’s light onto the black jewel on the very large box and were rewarded with a loud thunk. The box opened, and inside we found another box! This box was locked fast and bore two keyholes in its face, separated by a medallion embedded in the surface of the wood. 
Of course we immediately tried the silver key in each of them, but it would not turn.
Inside the leathern pouch
The party was at a loss, and we reviewed the information we had collected so far for some clue to the opening of this box. Our attention returned to (among many other things), the pouch Brigit had sent on her homunculus’ penultimate call. More particularly, we re-read the excerpt from Le Morte d’Arthur.
It was discovered that the pouch and the letter (or perhaps the key?) which it contained were an analogy for the scabbard and sword in the text — or so we surmised from our supposition that Brigit’s intended message lay in Merlin’s assertion that the scabbard was of more value than the sword. The pouch in fact had something to teach us. Turning it inside out revealed that a message had been written on the inside of it:
The left lock will turn if I press my hand against the medallion until the lock clicks.
The solution
Honestly, our party pressed more than one palm against the medallion, and even after a very long time, the box made no response, and still the silver key would not turn.
The box in fact needed no hand at all, only something cold, as Brigit’s hand was. After chilling the medallion for a while, a thunk was heard from the box, and the silver key turned in the left keyhole. It would not turn in the right keyhole, however. Heating the medallion produced another thunk, and the key turned in the right keyhole. At last, the box opened, and inside was a clear crystal cut into the shape of an intricate snowflake.
The conclusion
After the success of our treasure hunt, the swamp began to recede, and before a week was out, the roads were passable once more. Dash and I legged it to Yvelines, where we obtained a carriage to collect the others.
You might think there would still be some fighting to do over the inheritance, but to the best of my knowledge, none of the claimants has pressed the matter even to this day. Being stranded as we were was an experience which I hardly wish to revisit even mentally. I am content to pretend the old estate never existed, and I suppose that the feeling is common to most of the party.
Building the dowry box
Dad and I actually built the two locks for the dowry box ourselves. It took me several designs before coming up with something reliable that was easy to construct. We cut the frame and bolt from some thick acrylic Dad had lying about, and the bolt’s stopping points were controlled by putting a large compression spring and ball bearing into a slot cut into the acrylic frame. The bolt had a couple of bumps along its length so that it would snap into position when moving past the ball catch.
In the back of each lock was a ward which blocked the keyway. The ward could be extracted via a pull solenoid controlled by the dowry box’s brain, an ATtiny85.
Alas, the temperature-sensitive lock did NOT work on game day. Luther and I worked on it to no end, and we had it mostly working a hundred different times in a hundred different ways, but we found that once any solenoid activated, it threw a wrench into the temperature sensor’s ability to deliver reliable readings. (We used a TMP36 for the sensor.) We tried attaching capacitors of various sizes immediately against the sensor, but it made no difference. The solenoids were each only about 1.5 or 2 inches away from the sensor, so perhaps there was nothing that could be done with the intense magnetic disruption that their activity caused.
You can find the code for the dowry box on my repo at github: https://github.com/Vaselinessa/treasure-hunt-des-marais/tree/master/dowry-box.
The opera
A nod goes to Matt Crook, whose brilliant puzzle concerning the tombs of Caesar Augustus in his Blood Faith installments served as inspiration for the Atlanta-Atalanta riddle. Seriously, go read Blood Faith if you haven’t done so. It’s probably shorter than 30,000 words in all.
[1] http://javanese.imslp.info/files/imglnks/usimg/0/07/IMSLP19079-PMLP44870-HG_Band_87.pdf