Trying to install Windows 10 on early 2008 MBP without using the BootCamp Assistant
Just a thought, but can you install Win 7 using the bootcamp that is designed for your system and then once installed upgrade to Win 10?

Note that the Windows 10 bootcamp drivers will not likely support your hardware but I have had some moderate success updating bootcamp drivers on Windows installations from within Windows by identifying any unsupported hardware by their hardware ID (in device manager) and then downloading the driver directly from the manufacturer.