Finally I've cracked the last lock and freed the spirit of an unhappy young woman imprisoned there by an insane lover. You can see the final screen of the game below this paragraph (and the corresponding first screen above it). I was actually a) underestimating the lock mechanism (and I shouldn't have, considering the locks I'd seen before in that game) and b) concentrating far too much on the word instead of the keys. I'm not going to post the solution here, because it's too much fun to find it out by yourself.
Now that I know how to finish it, the game itself has become a lot more interesting than it was before ... because not being able to finish the game after having arrived at the last puzzle was quite frustrating. I'm quite good at finding all the objects right now (even though there's some things [and some English names] I have a hard time identifying) and so I've managed to make it on the top of the 'Best Agents' list the second time I finished the game (which was necessary, because I missed taking a screenshot for this post the first time I did): Agent Reet 02:23:45 (compared to the first time with 02:59:45). Little update: the third time I made it in 02:04:07.
That means I've finished all the games I bought online ("BeTrapped!", "Inspector Parker" and "Mystery Case Files: Ravenhearst") at least once right now. Maybe I'll go looking for some new games sometime in the future, but not right now.