You are right!
- 1 is faster (efficient) than 1,2, and 1,2 is faster (efficient) than 1,2,3, but
- 1,2,3 is safer than 1,2, and 1,2 is safer than 1.
That's the trade off. When you apply 1,2,3 combo, you won't commit your punch 3 until both of your parry 1 and parry 2 are successful.
I'd have to disagree with you there! I know this is a common belief among many martial artists. But parrying an opponents attack without delivering an attack of your own is always more dangerous, because you will forever be at your opponent's mercy. If you are not threatening him, he doesn't have to worry about what you're doing and can continue to attack you however he sees fit. The point of single time counters is, to use a fencing analogy, steal the tempo in your favor, and gain the offensive. An opponent who is concerned about getting hit is far less dangerous than one concerned with hitting.
Simultaneous attack and defense can be a hard thing to apply, as you've mentioned. But it absolutely works, and should be trained extensively so that you can pull it off. Part of the challenge is simply changing your mentality towards the fight, and focusing on responding offensively to any action your opponent makes.
As Lichtenauer says, "Note the word 'simultaneously'; it is the key to great fencing."