Criple. It provides the dot functionality even if it does next to no damage as a stamina build. The only stamina alternative is twin slash & morphs for a bleed dot. Undead and mechanical opponents are immune to bleeds.
That's a really good question. I have never used twin slash in PvP, so I can't really give you an answer. I do think they are susceptible to it, though. Maybe someone else can confirm.
Well, you could use meteor on your off bar. I use a two-hand, just for the rally buff and keep rending strike on my DW bar. So, I buff myself with Rally, Flawless Dawnbreaker and Relentless force for a total of 33% weapons damage buff. This increase both Meteor and Rending strikes significantly and if you have caltrops you can stack that with these also giving you insane sustained DoT with Surprise attacking weaves.