From what I understand, Nintendo removed one of the chips on the drive controller board which is necessary for reading GC discs. At the very least, a new drive controller board would be required, if not a whole new drive.... The usb support in dios mios will not directly mean that the wii remote and classic controller will become usable as well. This requires a functional bluetooth stack on top of the usb one. Since dios mios is running in gamecube mode, you can't use an ios to do the job for you. Even without looking any deeper into it, it looks like a dificult and serious amount of work.
Devolution on the other hand, is still running on a normal ios in wii mode (From the pieces I read here), so it should be easier to implement code to use the wiimote. Don't get me wrong, easier doesn't mean easy.
As far as the comparison of Devolution and DIOS MIOS goes, I had actually been considering starting a new thread about it. Above you pointed out a number of the relevant issues. Running in Wii mode under IOS means that the Wii's hardware is accessible. While it is great the crediar found a way to implement USB loading in Gamecube mode, that also means that hardware access is limited to what MIOS normally sees (plus whatever custom drivers are created).
Devolution:
- runs in Wii mode (meaning 1.5x GC CPU/GPU speed, regular IOS, access to USB, wi-fi, bluetooth, etc...)
- supports audio streaming for games like Ikaruga, Star Fox Adventures, Eternal Darkness, Wave Race: Blue Storm, etc....
- may offer somewhat better performance due to higher clock rates
- requires original discs
DIOS MIOS:
- runs in Gamecube mode (slower CPU/GPU clocks, custom MIOS, limited Wii hardware access, limited MEM2 access)
- no support for audio streaming (yet)
- should provide same game compatibility as DML 1.3, but faster loading
At present, neither is available for download, so it is unknown how they might compare in terms of user interface, stability, compatibility, etc....