At least Varys is very clear up front with Tyrion about what to expect. It was just back in Ep 02 that they discussed this possibility:

I suppose you could argue that Cordelia had started keeping and using her powers for herself, rather than just being a conduit filling the "vessels" around her, as Myrtle called them. And one little pep talk was all it took!

1.  Doubtful.  Myercella went down to Dorne already.  The Red Viper story is one of the most interesting (and tv-adaptable) in those books - even if they could excise it, it's unlikely they'd want to. 
2.  The PW should happen early in Season 4 - it's what sets everything else in motion.
3.  Apparently Belwas isn't any…

Yep.  In the GoT world, justice delayed doesn't necessarily mean justice denied - but it does mean the justice is both uncertain and may be realized in a way that doesn't satisfy a cathartic revenge fantasy.  Karma still exists, but works in a much more roundabout way - so it will be frustrating and less satisfying to…

Remember Cersei's philosophy - anyone who isn't a Lannister is an enemy.  She just presumes that everyone else is out to take what she has or wants, and behaves accordingly.  When the Tyrells conveniently ally, and then show up in town co-opting power, trying to marry the Queen regent, plotting to marry Sansa quietly…

Before Khal Drogo's death, she wasn't in a position to do a whole lot about Dothraki slavery - and she did what she could, intervening in small ways (which backfired horribly, of course).