Yep! I have all of the things above (yubikeys, many PCs, a couple voip numbers with SMS, ability to emulate Android on PC, a host of old smarphones...) still, i

Yep! I have all of the things above (yubikeys, many PCs, a couple voip numbers with SMS, ability to emulate Android on PC, a host of old smarphones...) still, if I lose the one smartphone on which -that- custom app is installed, I'm hosed. And I can't even install the app on a second phone, because: ah, ha! Only one at a time! There is no installing two, Luke. Only one there will be. (There you go... quotation inception.)