You should read up on lightning networks and Zerotime.   Limitations of blocksize  does not directly related this way. VNL can do 100's of tps  and Zerotime can add 100x's more tps without filling the blocksize as it is right now.  Along with this VNL is set to have pruning and scaling of the block chain in place.  Blocksize is never going to be an issue with vnl.

The security holes are inherent in all crypto.  In bitcoin a 0 confirm transaction is very very risky.  This is why companies that allow 0 confirm  (coinbase) require your info to make all transactions trusted.

Zerotime adds security on top of that by having nearly instant locking mechanism that runs through a group of incentivesed nodes.  As far as I know there is no 100% way to do 0 conf transfers, but the money and energy it would take to double spend in a lightning network such as Zerotime makes the  chances of it happening almost zero.

Think of Zerotime as a massive security wall made to allow you to make instant transfers in a trusted situation with zero risk, and in a trustless situation with massive protection.