I agree, replacing all M0 is 'optimistic'. Plus, even in the most successful outcome it's not at all sure that that's what happens—it could very well be GLO replaces a part of commercial bank created money (i.e. M2 but not M0) instead. The reason we use the number is to have some figure illustrative of the size of the potential. By basing it on M0, this max potential figure is not totally arbitrary, and it also allows for a simple back-of-the-envelope estimate since it assumes a world in which credit-based money creation by commercial banks would simply continue as normal.
Most important is that any level of adoption—as long as its self-sustained—leads to some amount of seigniorage that can be used for UBI.
Which part of verification seems cumbersome? Our goal is to delegate this to crypto exchanges, who already KYC their clients. The user experience of verifying for Global Income Coin will be similar to opening an account at an exchange, broker, or neobank (take a selfie, scan passport etc)