I mentioned this to a couple of Labour Councillors of my acquaintance, tonight.
They seemed to confirm my assessment that this is no threat to Corbyn at all.
They have not been fans of Corbyn until the General Election, when they suddenly realised that Corbyn (and themselves) could win under an old socialist banner.
Rather than alienating the Labour Councillors, the GE result rejuvenated them. They now realise that Corbyn is not toxic, and old Labour is not dead. And they are very happy about that because many local councillors are definitely old Labour.
So in conclusion, if a rule change did give councillors 4 votes instead of 2 in a 30 vote NEC, then it would probably strengthen Corbyn, not weaken him.