The Term : Rolling Checkpoints, where after a certain # of confirmations a Reorg is not allowed seem to block this issue outright and still allow a coin to stay decentralized.
Examples:
Blackcoin allows reorgs no deeper than 500 blocks.
NXT allows reorgs no deeper than 720 blocks.

My question is this:
Let's say their are no checkpoints , rolling or coded.

Someone buy the old private keys or at some point just actually owned over 51% of a coin total.

Say they try your attempt , but it was 3 months earlier when they owned coins.

The Blockchain has 3 months of confirmations ahead of them at a rated say 1 minute interval.

How do they ever catch up , with the block height of the main chain, won't they always be ~3 months behind?

* Now if you say it is possible to trick the time setting and somehow condense those 3 months into a day, please provide details or proof on how that is done.*
 

Thanks.