A CB point is locked in every 10 Consensus Days (CDs),
Each SH will be selected to provide a Transaction Block (TB) for a specific 10 second period during each CB. Each new TB will acknowledge the last seen TB, and the SH that created it will sign the CB along with a hash of the changes to the network state as of that moment.
Transactions will be typically confirmed in 5-15 seconds depending on network propagation lag, though if a TB is missed they may take an additional 10 seconds. As long as the TB chain is unbroken, these transactions are secure unless a massive network split occurs within the next 10-20 CDs before it has been accepted into the CB and signed by 100% of the consensus.
If I understand correctly, it takes about 10 days for a transaction to be secured by ~100% consensus.
It is hard to say what guarantees exist before that happens.
There is a trivial attack: a straightforward double-spend with shares you bought or hijacked. But, of course, nobody will wait only 10 seconds to confirm payment which has value above $3000. So it is not very practical.
However, I don't see how this system can survive network splits. Don't forget that shareholders can pretend that there was a network splits when there was no...
I believe Byzantine fault tolerant synchronization (it was mentioned by Ukigo) is a better approach... Basically you need to aim to get 50+% confirmation as soon as possible.
If there is a network split it won't be possible to achieve consensus, thus users will have a clear indication of whether payment they are receiving is confirmed is not.