In the case of 4, well, its just a disaster. Blocks can be replaced all the way back to the last checkpoint potentially and all transactions from that point could be destroyed.
You aren't capturing the realistic effect of a 51% attack. Society is not going to allow all historic blocks to be replaced. Politically impossible. Instead the viable attack is insidiously changing the protocol going forward, such as censoring transactions which violate some government edict, e.g. forcing every transaction to be stamped with its government KYC identification number (a la 666).
In any event its disastrous due to the agreement being based on resource expenditure and not the data set.
The 51% attack is allowed to go unchecked = centralized
Reliance on developer inserted checkpoints = centralized
Government intervention = centralized