Thanks for all the effort you have into this! It's certainly going to help me out since my car is experiencing the same symptoms.
This looks like an $800+ dealer repair...so people will want to test their own vent temperatures BEFORE their warranty runs out. For me that's going to happen by the end of this winter.
Before you open it up, maybe flow some hot water through it to see if if you can observe any obvious temperature variations or cold spots....thinking about the best way to open it up without getting metal shavings into the end tanks of the core.