My understanding is these instant confirmations are still recorded in the block chain and if they are a double-spends, they are not recorded in the block chain.
They might well be recorded in the blockchain, but what use is that to a merchant who accepted the deposit (at 0 confirms) and took irreversible action when the transaction was confirmed?
Since you can only spend on one quorum (or for instant x, then one designed masternode), then it is normally impossible to double-spend.
The double-spend risks comes from the holes in their design that I enumerated in my prior post(s).
I don't see how it's impossible at all. If I own a majority of masternodes, I can do whatever I like with my quorums and it doesn't just result in a 'no quorum achieved' it can result in double spends at 0 confirmations. Like I said before, if the system is designed to wait until 1 block has passed (in order to observe the quorum results), then you might as well throw it all away and just use POW?
I guess it is intended for low value casual payments like buying coffee. No exchanges accept it afaik.
Rather like accepting a transaction at 0 confirmations... except it gives merchants a false sense of greater security which is actually really bad in general.