Use USB-C cable as multiples regular copper wires
Could it be possible to use a USB-C Cable to USB-C Cable with both end controlled by me, just for use as simple copper wires, without negotiation, etc... Because i would not be using the USB protocol.


Not recommended. If you aren't using USB, don't use USB cables or connectors.


Yes, the power and ground pins are all connected together internally. You can't use them separately.
Depending on what specific type of USB-C cable you're using (there's more than one!), some of the pins may have pullup resistors, or may not be wired through.
Of the remaining pins, many of them will be made up of differential pairs. If you're using single-ended signalling, this is likely to cause you problems -- sending a signal over one wire in a pair will cause interference in the other wire.
Last but not least: if you use USB-C connectors, users will expect your device to be compatible with other USB-C devices, and will be surprised when that doesn't work. (And even more surprised if it damages one or both devices.) Users don't like these kinds of surprises. If you aren't doing to be compliant with the USB standard, don't use that connector.