I also have a DTSE9 and it’s been on my keychain for at least 10 years now :D

Recently I have just gotten 2 nvme ssd enclosures and have been very happily using them for the super quick image writes. I just checked if there’s some compact 2230 enclosures and some of those even have keychain attachment points… they definitely are a bit bulky for a keychain but I’d argue they still are an option and will look similar to a keyfob :)

Ideally you would never have to because you just have the two people come up with their part of the password and then initialise the LUKS partition together. Sorta like a key ceremony

You can also just split the password for a single LUKS into two parts and give one each to the two people :D

Honestly my recommendation would be to start off with some microcontrollers/Raspberry Pi and some breadboards and go from there. That way you can uhhh trial and error your way to something that works and worry about the “engineering” part later, when you have gotten your feet wet :D There are some simple Arduino starter kits for example that give you the basic things + a bunch of sensors and motors to play with. I got started myself with the Elegoo Arduino Starter Kit (~50€). Along the way it’s probably going to be helpful to look up some course on electronics basics, specifically digital circuits and then analog circuits if you’re feeling adventurous.