Regarding 2/3: ....Then the wallet would allow to automatically sign/broadcast atomic tx whenever my order is matched on a server?
Yes. The server will notify the wallet that it has found matches, and the wallet will verify they are valid then start the swaps with the other traders.
Sweeeeet!
Does that mean "instant transactions" like at the speed of Darkcoin (10 seconds)?
Or how long would the actual transaction take to verify?
No, you will still be limited by blockchain confirm time (but a green address model can be used to create instant transactions for trades). For small trades, it can be safe to use unconfirmed transactions, so the trades will be instant. For large trades, you will want to wait for confirms to make sure the other trader can't double-spend.