I may not be the best person answering your question and I'm sure the community will rebuke what I write, but I'll take a shoot at this.
1. Fairies tend to either pull aggro or bet hit in an aoe while casting. They can and will heal themselves though, but sustain is good for a quick regen instead of time taking to cat physick. I tend to use it.
2. In later boss fights dps isn't needed from healers, but an instant miasma II is a nice extra sometimes. Bane only hits 4 mobs at once while miasma II is infinite. You can replace it if you want.
3. Not that I've heard of. I asked it'll be fixed with next patch. I also hope they add a "follow" command too, but I'm just hoping.