:D
I had some trouble replicating the issue, but having the reports definitely helped me narrow down where the problem could be. Long story short, there are build differences between the web, mobile, and download versions of the game and some code that was meant for the download version didn't get deleted when I compiled the mobile version. When I finally managed to crash the main menu by backing out mid game, I was able to pinpoint exactly what the issue was, deleted the conflicting code, and recompile a new build! So in all, it wasn't tremendously complicated (thank god) but it was definitely a good catch on your part so thank you again for your eagle eye and supporting documentation! Also, you're correct! Ren'py uses Python structure to work which is great for me as I'm actually a novice programmer myself lol!
The monster endings are loosely based on leaning into choices that encourage the Love Interest to be more comfortable with their strengths, including their new monster forms. For instance, if you try to "run distraction" instead of getting between Marcel and the other demon at the at the climax, that adds a point towards getting the Monster ending because you made a choice that prioritized keeping Marcel from making an impulsive decision without rashly putting yourself between them. Basically, you're using your intellect, and that makes Akello more comfortable enough to let his monster form hang out so to speak!