Trouble tempering Valrhona Couverture Manjari (64%) Chocolate
A couple of thoughts...

Not commenting on Valrhona specifically but 12h at 131F seems like a very long time at a very high temp. Can you temper it on its own at a lower temperature? Cooling the chocolate to 84F without contact to a cold(er) surface won't seed it well. You can also try adding rough shaved tempered chocolate in sufficient quantities during the 82F-84F state.

What is the cocoa butter content of the Van-Heusen? I don't know anything about it but if it is extremely low, you may be creating a low cocoa butter chocolate that is difficult to temper.

Take all of this with a grain of salt (please don't add any). I worked in chocolate for 10 years but haven't for the past 15 and I may be losing my mind. These are the steps I would take if it were up to me.