IMHO WLP005 was the wrong yeast to use to achieve a fresh/bright hop character in this beer. Should do more homework...established experience/literature/audio commentary would have suggested this anyway
I'm doing my fourth attempt at Darkstar's Hophead (a similar malt/hop bill in quantity to this) on Tuesday, and the thing that'll make the biggest difference from III toward recreating the fresh taste of this beer will I think be the yeast.
From your Q's:
1. Next time out I'm shooting for BU:GU 0.8 so 30.4 IBU's in a 1.038 beer.
2. Yeast management-wise the 5L (or 4L as it actually was to allow for headspace in the 5L vessel) worked great. I've fermented out Whitelabs vials in 4L of 1.040 spraymalt a few times now and always been happy with the quantity and health of the yeast. Final grav in the finished 23L batches reached in 3-5 days (1.040ish beers).

Diacetyl = low, I worried unnecessarily
For the bright bitey zingy hoppiness to carry through I'm keen to try US05 in a single yeast beer of this type, but this next up HHIV will be by the book with and ingredient bill from the brewery direct (via telephonic pestering

). I'm posting for comment in the brewday thread shortly.
As a post script I tapped this yesterday for a BBQ...drew the last single pint tonight, so it went down pretty well amongst my mates.