I think we need to get the driver and fastboot command situation into a definitively working state to proceed. Try the following …
- Disconnect the phone from the computer.
- In Control Panel - Programs and Features:
Uninstall the Android SDK (it is not needed for your purpose) and everything with “MTP” or “ADB” in its name. - Reboot Windows.
- In Control Panel - Device Manager:
Enable View - Show hidden devices, and then uninstall every possible ADB, MTP and smartphone device, hidden or not. - Reboot Windows.
- Connect the phone again.
- Download the current Android SDK Platform Tools, not the complete SDK.
- Unzip the ZIP file to e.g. C:\platform-tools (you can unzip it to wherever you like).
- Add the path to where you unzipped the tools to the Windows “PATH” environment variable, so adb and fastboot could be run from everywhere without always having to prefix them with the path.
Then make sure the fastboot command works with your phone using fastboot devices and if so, try setting the slots again, without the space between --.