So um...
I kinda ran through the game again, and I told Henleigh about his second name manually. I got the key. Used it on the box. The key has been rendered as "missing" and appeared here.
Now the boombox is also infinitely pickup-able from that room where you get lost & found in castle. I have 3 boomboxes now.

In the second picture you see a Lost-and-Found-esque redemption pick up screen

And here in third you see the message that pops up when you Take it.

IMPORTANT - You don't get an extra boombox if you're holding one in your carry slot.
Oh, ignore my L33T cash. I was messing with RAM.
EDIT -
When I reentered the room the boombox on the floor was gone.

I've got 2 boomboxes now.
EDIT 2 -
This is ridiculous.

EDIT 3 -
Here's the dilly - I took the key and I only got one extra. I just went and gave the boombox to The One Who Raps A Lot and then I don't have any boomboxes here! The ridiculous amount in the vases is still there. =D

EDIT 4 -
(for PsySal)
Make the Property of Henleigh [Second Name] Red Box keyhole inventory a "directory" where the red key (an "important" item that has to stay in game and appears in lost and found) is looked for too.
EDIT 5 -
There's the red key again. This happened after I put the red key into the Red Box.
