1. Spamming ID is, in fact, higher DPS than True + Vorpal, so at that point in time, ID spam is worth it.

2. True + Vorpal + Full is, as you say, superior to ID. When you FT, you stop using ID ... until ...

3. At level what, 44 and 50 or so you get Disembowel and Chaos Thrust which chain off of Impulse Drive. These attacks are rather good attacks. The ID-DE-CT chain is excellent.

4. Flank (I assume you are referring to Heavy Thrust) means you are attacking the target from the side.

4a. This is usually applicable to encounters with multiple players where you get easy access to the enemy's side.

4b. You can consistently get flank attacks while soloing if you move quickly.