I've played through Carcosa many times. I've tried with all sorts of investigators and combinations.
The last scenario's set up has three versions, depending on your doubt/conviction. I've done doubt runs, I've done conviction runs. I've played those two scenarios a lot. But the third option, have 5 or less total doubt/conviction, I've never seen.
I Just played through, trying to get it. I tried so hard to avoid any doubt/conviction, always aiming to get neither when given the option (like fleeing the dinner party, killing the Oath breaker in Echoes, etc.)
Even trying, I still got 6. One from Curtain Call's ending, 2 from the epilogue of Unspeakable Oath, 2 from advancing the act deck of Palid Mask, and the final one from advancing one of the decks in Black Stars.
Is it impossible to get the third variant without dying in a few scenarios? It seems impossible to get the third option unless you really don't know what you are doing.