I am by no means a pro, but from experience; the first thing you should do is go check out the place where you'll be shooting the portraits. How high is the ceiling, what colour is it, what colour are the walls, are there any windows where bright light is available? Then see if you can test your set up yourself before the real deal. Can save lots of time and embarrassment that way.
What you have in mind sounds great. Just make sure the background isn't too dark, otherwise the office might start looking like a dungeon.
If there's a bright window, a white sheet over it can do wonders. For off camera flash stuff, the strobist group on flickr is the best place to go. Search that place for loads of ideas and diagrams etc of DIY softboxes etc. If you don't make a softbox, just bounce it off something.
I'm not sure what you mean by shooting f/6 to get a blurry background. Wouldn't the background be in that inbetween state of being blurred and somewhat in focus at that aperture?
This is at f/2.8 with a 50mm prime (D70s, picture uncropped). You can still make out the stone wall in the back. I guess it's a good idea of what soft natural light might look like (it was an overcast day).
Here's another example, at f/1.8
Technique besides, I think how you would compose the photos around what you have in the office would be more important to the end result. Going there and checking the place out would help with that too.
Don't worry about it too much, I'm sure you'll do just awesome 
You know, if your UW strobes work as optical slaves, maybe you can take some a long if you need some additional light (no ambient, background might be dim, etc). They'd look weird but they might help you get the job done.
