@Nymunariya
Nothing but the ROM file has been modified (and ATI drivers, if on v8 and earlier). As such, a lot of the problems you detected are unrelated to any of this (like the suitcase issue you had).
Any language OS 9.2.2 installation will also work fine, as long as it doesn't require different assets located in the ROM file. But if it does, that has been done before: Some pages back, there was someone who was instructed on how to "merge" the ROM with a Chinese ROM via IIRC ResEdit, for example, so even such cases are doable, and, if ever needed, you may consult that for help.
Sound works perfectly via a USB audio adapter (mentioned and linked to many times by now).
Graphics work as perfectly as they do on any other Mac with that GPU or an equivalent GPU.
A USB hub works fine for the task (and what you used seemed like just another USB hub, with less ports), as long as it's externally powered, as well. The "mouse bug" is, as far as we know, completely unrelated to the usage or lack thereof of USB hubs. (It happens with or without them. A restart sometimes will and sometimes will not fix the issue. Most of the times, it does.)
Getting OS 9 recognized on OS X has indeed been asked about before, too, and that requires either spoofing the machine ID to something OS X acknowledges as a Mac-OS-9-compatible Mac, or hacking the Startup Disk module of OS X (maybe editing .plist files could do it?), so as it is, alt/option-booting is the only currently-available option to switch between them.
For the record, all Mac mini G4s are also technically 10.2.8 Jaguar compatible (no GPU acceleration) and 10.3.4+ Panther, besides 10.4 Tiger and 10.5 Leopard. (The switching issue persists across all of them.)