Any other ideas of how to scale a crypto-currency to beyond 10 million users within 3 years or less from launch?
Build it into a game that becomes a massive hit. Unfortunately I don't know any way to engineer a game that is a massive hit. They all seem to be black swans. Most often designers of very successful games who try to repeat their success fail and end up being one hit wonders. Maybe that is not universal though, I'm not sure.
Installments of an already-successful franchise can get wide distribution but you will need to pay for that, so it comes back to advertising (a form of product placement in this case).
Have it stamped official by a government.
That might work, depending on the government. Official approval by Tuvalu might not do much (though it arguably could).
Have it backed by a major corporation. Like for instance any major fast food or coffee retailer. Possibly an industry as a whole.
Get a celebrity or team of celebrities to not only endorse but actively promote.
In all of these options killer one click apps for phones and computers are a must.
These all come down to advertising really, unless the celebrity happened to take a personal interest and made it into a hit for whatever black swan sort of reason. But celebrities are so managed and used as promotional tools these days that almost everything they do seems to be some sort of product placement deal.