So Hank became a drug kingpin, committed murders, etc for $200,000? Where's the rest of the money? Right now, Hank really has no motive for being Heisnberg unless someone could find millions of dollars hidden away somewhere (I'm tempted to do one of those lame ass "oh wait" takes here).
This is the reason, I think, Hank initially referred to Walt's confession as a threat. There is no evidence other than Walt's word that Hank is Heisenberg.
I took the 'nail in the coffin' remark to be a reference to Hank's career if the video were given to the DEA. Because it would be highly unlikely that Walt's 'confession' alone would be enough to even get a case to trial against Hank, despite the $177,000 rehab payment. But the video would certainly place Hank in a position where his personal integrity could become an issue and that alone would probably end his government law enforvment career.
I think Walt's main intention with the video was just to "muddy the waters" enough that Hank might decide it would be more trouble than it's worth to continue pursuing him. Even if the DEA did eventually see through the ruse and side with Hank, it would take a lot
of time and energy on his part to get to that point. And in the meantime he would have completely trashed his career.
Plus I think Walt wanted to show him just how
good a liar and actor he was-- something which I don't think Hank truly realized until he saw his performance in the video.