Thank you for your thoughts, Robin!
So, you are suggesting I use two other arduinos to communicate with my Pro Micro? Sounds like the way to go, since the push-button-device and the acceleration-sensor need some platform to communicate through I guess. That means two other little boards, which sounds okay, since I am sure they can be lightweight (simple, I mean).
But what wireless technology to use for each of them? How (by which wire(less)) do I get them to communicate with the Pro Micro - in the easiest and cheapest way?
Infrared for the buttons and SOMETHING (for the acceleration-sensor?). 433 MHz for the buttons? (e.g. buy one of those cheap 4-push-button-devices on 433 MHz and teach my Pro Micro to read those buttons?