Ah Talmid,
Obvious when? If it was obvious, at that time, (say if there was a gun to his head), then you would be right. Because there would be an anan sehadi as to his daas. But if you are basing your “patently obvious” on later behavior, then it cannot be mevatel a geirus retroactively because of dvarim sh’blev.
You are also assuming you have an “anan sehadi” but the Talmid Chacham overseeing the process does not. That is why I cited the teshuva from R’Moshe. If the Rav is knowledgeable and frum, we can be machzik that the kabola of mitzvos is valid and the convert is a full fledged Ger!
I would suggest learning the sugyos on giyur including some teshuvos over the centuries and then coming back.