I'll go with Beria as well. In 1942 the NKVD was powerful enough to simply disappear anyone who opposed him.
In 1942 Molotov was still well-connected with the Party bureaucracy, but I don't think he had either the charisma or raw political power to have grabbed the top seat and held it.
> Molotov is underused
Yes. Stalin made some bad decisions, then blamed Molotov for carrying them out to the letter. After Stalin died the Presidium found it convenient to blame Molotov for various blunders in diplomacy during the war.
Still, had things been a bit different... Molotov was a shrewd judge of character, a successful negotiator, and on a first-name basis with most of the USSR's friends and not a few of their enemies. As far as I can tell he successfully carried out every task he was assigned, even whoppers like making an alliance with Nazi Germany. Even now, that sounds like a ridiculously unlikely scenario...