LotR -- Book 1 - Chapter 06 – The Old Forest

[Formendacil]

Originally Posted by Legate of Amon Lanc

Nevertheless, on top of everything: Does Merry actually take this one key with him somewhere abroad, to Isengard, Gondor...? (Okay, he probably loses it somewhere along the way, but maybe at least as far as Rivendell?)

Maybe, but as I read your post, I was given to suspect a different answer:

Originally Posted by Legate of Amon Lanc

Last remark belongs to the often forgotten Fatty Bolger.

It seems most likely to me that Merry would have left the key with Fatty--not least if it was either a.) one of only a very few keys and/or b.) borrowed from someone else.