If you have 100 NXT in your account, that means you have 10 NXT for "instant transactions". In order for you to "double-spend" (literally to spend 200 NXT, so 10 NXT * 20 times), you would have to fool all the peers on the network 20 times. Chance of that is so low, in most cases you will just end up paying the merchant multiple times
In the scenario of A sending to unconnected B and C, because he has 100 NXT in his account, he ends up sending 10 NXT to B and 10 NXT to C, no ripoff there.
Chance that all 10 merchants r isolated from Nxt network is below zero, I think.
Chance of peers being isolated is below zero, but the chance of all peers being able to communicate with all other peers to prevent double spend is even lower than that. Plus, the doublespender has a one minute window with BCNext's proposed method, his balance would have to be updated in the blockchain for doublespends to be prevented.
Hm... my example was wrong.
Sorry if someone mentioned this before, but if the merchant receives an instant transaction from the special account, it can broadcast it to the network (connected peers) to check to see if this account has already sent this type of special transaction in the last minute. Those peers will then rebroadcast this request, and so on. If a match is found, then the peer who has a match will send this information back to the merchant. Assuming a latency of 200ms, this request can be rebroadcast 5 times in 1 second. So if the merchant is connect to 10 peers, each who is connected to 10 peers, etc. then the network can quickly check 100,000 peers in 1 second. Not to mention, some hallmarked nodes have thousands of peers, so this number can be exponentially larger.
If a match is found, then the merchant can just say, "sorry, you already sent an instant transaction in the last minute, please wait wait one minute for confirmation." So time is not an issue unless the merchant is completely isolated (which is nearly impossible). Most humans will never need to place a large number of small orders in a span of 1 minute from multiple sources, so this is not a limitation. If they really need to, then it makes more sense to use their normal account (with 100% spendable funds), and just wait 1 minute for the confirmation. Instant transactions are useful for things like grocery shopping, I suppose. You're not going to checkout 100 times in a minute.