iMac - Charging Magic Mouse & Keyboard.

There would be zero advantage to do that, it's best to follow Apple's directions and use the Lightning cable that came with the Mouse and Keyboard or any other Lightning cable.

Sorry, I have no idea however I suspect it is possible based on Apple's description in:

https://www.apple.com/shop/product/MX0K2AM/A/usb-c-to-lightning-cable-1-m?afid=p238%7Cscgkd8wD5-dc_mtid_1870765e38482_pcrid_246386725857_pgrid_14874603490_&cid=aos-us-kwgo-pla-btb--slid---product-MX0K2AM/A

Yes, they will charge with a USB-C cable. The iMac must be on or sleeping. It wit not charge while the iMac is off.