What you're describing is not a checkpoint then. A checkpoint forces the identity of the selected chain, regardless of it has the most work or not. So that would be the point of departure.
Indeed I see where the misunderstanding has been then. Perhaps check-point was the wrong term but it sure has a "check-point-like" feel to it. Glad we are more on the same page now.
Essentially what I'm trying to figure out is a mechanism for blockchain compression so that we can drop very old txs with minimal to no loss in security. Perhaps what I have proposed is not sufficient, I merely thought of it yesterday and thought we could explore something like it here. You're right, the complexity of such a protocol may simply not be worth it. But consider a new node in 50 years having to go back to genesis and start validating all those txs. Poor soul.
In a certain sense, it boils down to resetting the genesis block to something more recent (of course including the utxo hash) in a manner compatible with the network's consensus. Do you suppose there is any secure way to do this? Would it even be worth it?
and no, newly generated coins cannot be spent for 100 blocks
Right. I forgot about this.