If you'd be so kind, please answer the basic question: do you have a suggestion regarding how to cement a checkpoint block (that cannot be reversed by 51% attack) in a way that doesn't require proof-of-stake?
With LDD WBC, there is no need for a specific checkpoint block, every block becomes a checkpoint after it is buried deep enough.
Say we use the last 5,000 blocks as our LDD metric. The average LDD in each block is say, 1,000.
You, an exchange, are interested in a transaction in block 2,001.
Block 2,002 comes along with 1,050 LDD.
Block 2,003 comes along with 800 LDD.
Block 2,004 comes along with 1,500 LDD.
Block 2,005 comes along with 1,110 LDD.
Block 2,006 comes along with 900 LDD.
Block 2,007 comes along with 1,200 LDD.
Block 2,008 comes along with 1,500 LDD.
Block 2,009 comes along with 1,300 LDD.
Block 2,001 becomes cemented because there are 6 blocks that follow that meet or beat the average LDD. This is a client-side operation and the software must notify the user if a competing chain attempts to reverse block 2,001, but it will NOT automatically replace it. In reality, litecoin would probably use 20-30 blocks before cementing since its block time is 2.5 minutes instead of 10. An hour should be more than enough time for that block to have propagated the network. If you're extra paranoid, you could wait until block 2,009 is cemented.
If you do not have faith that blocks will be properly passed around to achieve this, then you can't possibly have faith that the proof of stake signatures will be immune to the same problem. If you're worried about some miraculous chain of events that might cause a significant, temporary fork, then use more blocks to be safe. You have suggested 100 blocks to be a checkpoint for PoS, the same number could be used for the LDD WBC. But I think it's overkill considering, AFAIK, bitcoin has never gotten past 1 orphan block. I do not know about LTC, but it would be interesting to see since it's 4 times faster. Is 1 block still the biggest orphan?
Days destroyed weighted block-chain advantages:
* Clients have a say in the matter. Every client. Miners are forced to include every transaction possible because if someone else comes along and does them one better, their block may be invalidated. Even if only a single miner is doing the right thing, the clients will be using his chain over a malicious one. So as long as one miner is honest, the honest network wins.
* Clients have the power to choose which block-chain is the correct one, not basing it off of hashing power. This is the ultimate blow to any 51% attack. Want to create a monopoly? Oh well someone else came along and is offering cheaper tx fees, goodbye.
* Absolutely no additional data is added to the block-chain. Nothing to keep track of except mini-forks which may be slightly more likely depending on how the final algorithm works.
* Money is given no more power than it already has. There is a veritable check and balance system between clients, miners, and the wealthy.
* Difficulty CAN GO DOWN without opening the network to attack. This means transaction fees can go down. And stakeholders don't have to be paid to cancel out this effect.