1. If mouse support comes, I'd be surprised if it's limited to USB.So you need usb a to usb c dongle
2. Not all iPads have USB-C. But for those that do have USB-C, I'm sure somebody will build a USB-C wireless RF mouse if Apple limits mice to USB.
3. For the other iPads, you can use the existing Lightning to USB-A dongle. That would also work with wireless RF mice.
4. USB-C to USB-A adapters are very small and cost about $3.