Wow, thanks for the nice words, intric8. :)
Yep, on an unaccelerated NTSC system there is simply not enough raster time in one frame to blit all objects. The game is definitely optimized for PAL system, with the option to play on NTSC added later on with the recommendation to use some kind of acceleration if it's a vanilla system. ;)
I will have a look into those graphics bugs. Seems they sometimes appear on certain configurations. It's a bit weird.
Anyway thanks for the detailed feedback and for even documenting it! :)
Hope you will go on having a blast with the game with your son. Love playing games with my 8 year old as well a lot..
Greets,
Michael