Congratulations. (I'll get that solution amended too so that it works with the Spectrum version... I'm guessing the command is needed in the C64 version too, but I will check)
Yeah, it is annoying when you can't go back to get something you missed but expectations were different back then. The sort of player who was playing one of Simon's games would have been quite willing, and quite used to, restarting games to check they hadn't missed anything earlier on. Nowadays we sort of expect to be able to complete text adventures without having to roll back our progress.
You'll often find "point of no returns" in Simon's games and other Quilled adventures, because Quill had a very limited number of flags available to the adventure author, so you needed to put in place one-way points so you could reuse the flags for other things later on.
It's also interesting looking back now to see the "trends" in adventure gaming, particularly in the homegrown scene. We went through a phase, for instance, where looking under, behind, and around things was quite common. Or the phase where examining or searching things more than once was necessary to retrieve every item. A lot of these phases can be quite frustrating when you're not tuned to the correct frequency.. even if you were the one that wrote the game.