you shouldn't care about your game being turned back to source. anyone passionate enough will be able to read the assembly itself and lift it up to usable code no matter what the language or obfuscation.
iirc i think it is yeah, reverse engineering code even clean room isnt really considered legal, as you dont own rights to said software to do so, but im no legal expert, ive just heard stories of people being arrested for this
C/C++ vs C# for RE protection is largely just people squabbling over semantics in my opinion (as someone who did a ton of old Xbox/Xbox 360 RE and has experience in other RE projects).
Clean room is explicitly legal in most places. ReactOS and Wine would have been squashed by Microsoft had any part of their operations been illegal. :)
So long as you do not distribute their code in any form, only send patches or use clean room documentation you are pretty much clear. Your own patches are your own IP.