But a more subtle attack is to only produce the block for 50% of your peers and not give it to the other 50%  *AND* to do it at the last possible moment such that some peers include it and others don't.

I am not expert, but I remember BTT thread about how Nxt works:

Let's say I have 25% of NXT coins. And I want to mount an attack. I need to do the following.

1. Wait until I am selected as a forger.
2. Create 2 blocks, one for the network and one I hold back.
3. Continually add more blocks to the block I hold back. This is my chain I will introduce later as my attack.

The problem is step 3. To add another block to my held back block I need to be selected as the forger for that block too. However forger selection is based on the hash of the previous block and my account address.

Neither of these I can change quickly enough to be sure I generate the next block. So my probability of being selected to build the next block is 25% for each block.

2) What if you are a new node and you first connect to the network.  You see two chains, how do you decide which chain is the 'best' chain? 

Is this related?

https://nxtforum.org/news-and-announcements/economic-clustering/