I'm excited about Flarum but my initial impression is that you have made a critical error in your business model and tied your Kickstarter campaign directly to this error. The global username thing is just not a good idea. Imagine I am Rick Beerdog from Ubercorp and I want to setup a hosted plan for our support forums. However, I find out I can't use my own name on this service we are paying for because someone is already using the name on another instance of the software. I also find out that this username conflict is likely going to impact several of my employees, and likely many of our customers. You just lost a sale.
Since the Kickstarter essentially pre-sells these user names, you can't make the decision to back out of this problematic feature. I'd like to hear your comments on this issue.