It's not that bad. If the larger block miners are >50%, they will build off the longest valid chain, so they will ignore the smaller block miners blocks since they have a lower difficulty. If the smaller block miners are >50%, they will always have the longest chain and no large blocks will ever survive reorganization for more than a couple confirmations. Block headers contain the hash of the previous block, so once your chain forks, the blocks built after the first split block are not compatible with the other chain.
No "longest valid chain", all of the nodes which have not adopted your Bitcoin-prime will reject the >50% hashpower's "invalid chain" to the 'true' Bitcoin network those miners will simply stop existing. From one currency you will have two. It is a maximally pessimal outcome at the near 50% split, and it wold be against the issue of any Bitcoin user to accept a non-trivial risk of that outcome no matter what the benefit.
I'm not sure I understand the "No". As far as I can tell you are agreeing with me, but your notation is confusing me.
I was just refuting his claim that bitcoin prime miners would accept the blocks of the bitcoin classic miners by explaining that blocks wouldn't be compatible between chains since they have to include the hash of the previous block.