1st question: You don't want your enemies to attack your phantasms. You spawn a new after a shatter, follow up with 1-2 clones and shatter again. If they attack them still, you're largely happy that they have taken some hits for you.

2nd question: It's decent but not that great. Remember that the dangerous conditions builds overwhelm you with it. Just take this very one and how many stacks of confusion alone it generates. Plus things like daze don't even count as conditions. Hence I'd still to Null for its versatility and suffer through the rest - can't have everything in one build.

3rd question: The core traits are those that I've taken "first" in each line, but in the end it's personal taste and what you want to achieve. It's always gain some, lose some - my selection is based on my personal opinion where the gain is largest and the build is most coherent.