The only reason to worry about legal stuff is if you plan to sell your OS...
If this is the case, then I would worry more about the legals related to using the Windows Driver Kit more.... pretty sure those would be restricted as well....
That's why I'm looking at clang or another compiler - since the WDK also has restrictions.
If I have to I'll change the first stage boot loader to make it load ELF and change the second stage boot loader, kernel, boot and other drivers to ELF and then write DWARF version 4/5 debug support into my boot/kernel debugger.
But was hoping to avoid that if possible
I don't have plans to sell the OS (currently - and may never do) but was considering building it for a university level course at some point. For that it would be best to have it be utilizing fully legal software.
You may wonder why I'd would do another OS, well, I spent 15 years working on Windows and there are a lot of things I like about it, but there are also things that I feel could be improved upon and which would be easier to improve in a new system.