Preparing for my first baby shoot

Please ignore my previous post (unfortunately it's not possible to edit or delete it)...

Well, the shoot is over and I think I got a few good shots although nothing like the amazing ones I found online and posted earlier. Still, the client is (so far, judging by the camera's LCD preview) happy, and I still have PP to do which should further improve them. Though not entirely happy about it myself I did learn a lot though!

1. age matters: the baby was 3 weeks old, and I now understand why they should be younger than 10 (or 14 at the most) days for best results. Yes, there was a lot of sleeping, but not the very deep kind which allowed us to do all those amazing poses
   
2. lenses: KISS (Keep It Simple, Stupid) -I brought all my 3 lenses but ended up using just the 35 f/2. Shot mostly at 5.6 or 4 to ensure in-focus shots
3. lighting: my biggest challenge in this shoot. 2x Canon Speedlites. A stand with a shoot-through umbrella for the 580EX II (triggered manually using a PC-sync cable and a 430EX used as a fill-in flash when needed (optically triggered by the 580EX II. Didn't work very and cumbersome. Perhaps a softbox would work better (I'm considering the Lastolite EzyBox Hotshoe 21"x21" or 30"x30"). I thought an umbrella spread the light everywhere (making for a very soft light) but apparently it was very hard to determine the lighting and to control it. The room had some large windows, but it was a sunny day and harsh streaks of light came in so I blocked parts of it and also partly closed the blinds which helped. To make things even harder there were clouds which changed the ambient lighting all the time. I ended up using mostly flash and very little ambient light, but found it hard to set up and control.

Luckily I might get an opportunity to shoot a newborn (less than a week old) in a couple of days, giving me a second chance and more practice. What do you suggest I do about lighting if ambient light isn't enough (I chose not to open up the aperture too much and I'm worried about using very high ISO values because of noise. How can I improve, ultimately getting closer to the kind of shots I posted here earlier?