(or maybe the v1_2_15 build you uploaded to itch is outdated and isn't actually the final Final Columbus :P).
...this might be the issue. That would also explain why you weren't able to find the entrance just exploring normally.
Viewing post in small graphical inconsistency (almost fixed)
Oh my goodness, that was it. After finding out that you weren't kidding about how the game gets buried in searches (even with the "Heart of Ruin" added to it), I stumbled across the build you uploaded to Game Jolt, which is 1.2.19 and includes a bunch of other changes, like an existent room 241 and a combo mechanic that shows how many times in a row you hit enemies. Any chance you'll reupload that as an update to Final Columbus over here? EDIT: If you purposely chose to revert back to 1.2.15 for some reason, maybe you could give people the choice of which to download? EDIT 2: On second thought, maybe it'd be better to stick with version 1.2.15 since the upgrade for the Chain Core wasn't implemented (the sidequest seems to stall after giving the Master Floppy to the shopkeeper), meaning you don't get anything for fighting your way through that nonsense. EDIT 5: and the new boss fight against the electric-gun-wielder crashes the game when you try to speak to her afterward, so there's another reason to stick with 1.2.15. EDIT 6: and 1.2.19's credits are missing the comments/descriptions for the enemies/staff, but it does list all the enemies that were missing in 1.2.15's credits.
I guess I only have one more question: 1.2.15 has an unused music track called "sound87" that isn't present in 1.2.19 at all. What's the story behind that track? Was it a prototype of Frozen Hearts, a track for another one of your games that you considered reusing, or something that someone else composed and you included by accident?
I don't remember the exact details, but "massive stability issues and unfinished content" does sound like a pretty compelling reason to not go with that version. Presumably version 16 was what added the new stuff and 17, 18 and 19 were bugfixes (I have no surviving documentation from those days, it was before I even started using source control).
sound<NUMBER> is the default filename for a new sound asset before it gets properly named, so it was something I added and immediately forgot about. Back in those days I usually had songs on repeat in the GM editor instead of having a separate media player / youtube playlist for music so it might be an intentionally unnamed song I just replaced whenever I wanted to swap it out.