It wouldn't surprise me if they insisted people be registered, accredited brokers before they're allowed to do anything like day-trade one national CBDC against another.
Nope. Fiat currencies, broken though they may be, aren't a ponzi scheme. There won't be any benefit for adopting early. You're just surrendering your privacy and autonomy faster by doing that. They will retain the same value as the existing national currency. In other words, you can't profit by selling your digital national currency back for the same physical national currency. All it means is that cash will be gradually phased out as more people adopt the digital version.