I
think I can help with the bit to Versailles, I've spent enough time studying the RATP site!

You need to take the RER C to Versailles Rive Gauche and it make sure it's Rive Gauche as that is the closest entrance. We're getting on at Champ de Mars Tour Eiffel and it takes about 25 minutes.
When I've put the route into the RATP site it does say this
As of Monday, July 16, 2012, every day because of work, traffic is stopped between stations Gare d'Austerlitz and Invalides on the RER line C until Saturday, August 18, 2012. (using google translate)
and tries to tell us the bus will be quicker (at an hour and 11 minutes!) but I'm hoping that as the station we're getting on is further down the line than the disruption that we'll still be OK to go by train.
I estimate it's going to cost 8 return each and as I don't think any of my DDs qualify for cheap travel I think we're going to buy a carnet of 10 tickets to save a few euros.
If anyone thinks I'm giving the wrong info here please say!!
Have you bought your tickets to Versailles yet? I've been on the Trip Adviser Paris forum where people strongly advised buying them as apparently the queue for security is bad enough without having to queue for tickets too so DH and I have bought ours online (the girls are free for the chateau).