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.