I've not got any significant experience of C but I can easily imagine there must be a whole bunch of code pitfalls for a game of this age, especially when it's been through the hands of multiple maintainers over many years. I find it bad enough trying to debug my own GDScript in Godot, so I think I'd stand no chance wading through history like you :p
The font is great! A bit tiny at 1440p but I can always tweak my res down while playing. If only there were a way to scale fonts in the same way as you can with graphical tiles...
Hope the save is useful and helps prove your fix works - good luck!