Brangdon,
Why do you assume that lending will be something everybody will do?If you have sufficient coins that leasing pays for itself, it's the rational thing to do, with no downsides, so most people will do it. (Except the ones that have enough to justify running their own node.)
Why only about 20% are forging then? The downside is centralization of power. If at any point of stake history anyone had 50+% of current stake rate (i.e. current is 20% means that lending to a pool more than 10% at any point) it will haunt the coin forever.Why do you assume that it will not backfire giving one person extreme stake history who might not care about the coin at the present?It's something we need to be vigilant about, in the same way the Bitcoin community needs to be vigilant about mining pools becoming too powerful. However, because Nxt forging doesn't have the same economies of scale as Bitcoin mining, there is less pressure towards centralisation.
There is nothing you can do other than checkpoints. For PoW is one time event that doesnt haunt bitcoin in the future. Not that Ilike litecoin but Litecoin had 51% on 1 pool lately but that will not affect its future if the hash distribution changes. For PoS it does.Why do you say lend your coins to a person you can trust if you want to build a trustless system?Because trusting a forging pool is no worse than trusting a mining pool (except you have a choice about which pool you trust, in Nxt.)
See the difference in first point.All PoS coins are very centralized in their stake distribution and dont forget that when you say they are decentralizedInitial distribution is orthogonal to PoS. It's a problem for Nxt, partly because it's so new. It improves over time..
Dogecoin, darkcoin are newer but better distributed.. Not an excuse. A serious PoS algorithm that wants decentralization should be as distributed as possible from the get-go because of security decentralization and history attack. Is that because of transparent forging? Could you explain how NXT might be more immune?
25% of active stake is not enough, you need majority.
May not be enough for reversing the chain but No 25% is enough for double spending. In fact it enough for even bitcoin. The difference is that for bitcoin it will cost money but for PoS it wont... You can double spend a casino as many times as you want given enough time.