No, obviously you need to do the same or more total hashes as the real chain (it is a 51% attack...)
The "bad" part is that you can make your chain have more blocks while having the same start and end nTime.
And yes, it *should* be using 3-7, 7-11, ... but it doesn't. (probably to avoid the issue of the first interval after genesis, as you'd need to know when hashing of genesis started = the timestamp of the block before genesis).
The code I'm currently playing with gets around this by special-casing that first retarget to have a nInterval-1 span instead of nInterval.