Oh wow, Wulden! That's super awesome! And thank you for the complements on the game! I always felt like the Zapper was underutilized, so it was fun trying to breath a little more "meat" into a Zapper game.
1) Regarding the score, I think the behavior after 65,535 is a bug. I wrote the game in C and compiled down to assembly. Ostensibly, score is stored in an unsigned long and there is some code that caps the value at 9,999,999, but I haven't closely analyzed that assembly produced by that code.
2) Danger Levels are uncapped. The limitation, as you observe is basically human limits versus the time limit.
If you'd like, I'd be willing to share the source code with you to help do your achievement setup. Email me at robbie@skyboygames.com and I can set you up.
Hey Skyboy! I am at present putting the finishing touches on the achievement set: https://retroachievements.org/game/32821
The only achievements I think I am missing that I was hoping to figure out was trying to get a counter for "stopping enemy attacks" I was hoping to challenge folks to use that more, but it is what it is.
I am hoping to have this set launched by the end of the day today, so hopefully it will drive some eyes in your direction XD