Thanks for the reply!
A 32u4 board fits the project really well as it has the USB transceiver, and as the Feather has both BlueTooth LE and built in battery charger it seemed like a good choice when the need is to emulate a keyboard on both USB and BT.
For the power, I think I will power the other board, probably a Pro Mini, from the battery so it will also work when the prototype does not receive USB power. Do you think that will be a problem?
I also think the bulk is libraries, but the switch cases do take up a lot of space - as far as I can calculate around 5% each, amounting to 25% of total program space.
All I need at this moment is a working prototype that can be used for a couple of weeks. I really could not have come much further by now, as our thesis is much more about the development process than the prototype itself. There have been a lot of complexity added along the way that we could not have foreseen.
I am not American, but I am a student, and this project is for my thesis 