Scanning a QR isn't actually necessary. That flow exists for when you open the invitation on e.g. a desktop or laptop, and you actually want to set up Snikket on your phone/tablet. The invitation page guides you through you what to do in that case, you don't need to know what a "QR code" is. But most people just open the invitation directly on their phone, and it has a magic link to install the app.
In some ways it's easier, because you don't need to go through the SMS verification and stuff that the phone-number-based platforms require. I know various APIs and hacks have made that flow easier over the years, but it's still an extra step for non-technical people to get lost in. It's also an obstacle for people who want to use a tablet, such as my children who are too young for phones.
In any case, I'd encourage people to be less defeatist about the adoption of more open alternatives (intended or not, that's how I read your comment). It is possible... my own family average no more or less technical than any other. They manage to use WhatsApp without help, and they manage to use Snikket without help. And of course I'm not the only person using it successfully.
I think the best bet would be to create a device with the sole functionality of establishing an encrypted voice line to you. Essentially, a plastic brick with an on button, speaker and microphone. No screen, no UI, just on/off. Like a walkie-talkie, but for infinite distances.
