I would probably chuck the reflector and use a white bed sheet as a diffusion panel. Hang the bed sheet such that it acts as a large softbox at one corner of the bed. Some of the light go through the sheet and act as a soft diffused light similar to a softbox. The rest will bounce off it and head toward the wall behind it and bounce around the room as fill.
I would then use the 420EX as an accent and fill light. By that I mean I would position it off to the rear/sides, or directly behind your GF so that some of the light hits her directly and creates a highlight, hair light, or rim light. The flash should be pointed so some of it hits her directly, and some of it misses and heads forward of her toward the bed sheet, walls, ceiling, etc, for bounce/fill. You will need to make sure its hidden behind her from the cameras view.
With that setup, you should be able to get a soft diffused light on your GF, with some harder light as a rim or hair light. For the most part, the only light you will have to move around would be the 420EX to keep it hidden from the cameras view depending on your cameras position.
The 420EX will dictate the exposure since it will be full power as you mentioned. The SB800 should be adjusted to be within about a stop of the 420EX, either overexposed to subdue the effect of the 420EX, or underexposed to enhance it. If you can get access to the window, you can even put the 420EX outside the window, and reduce the SB800 about 2 stops to get the 'sun through the blinds' effect. The SB800 at 2 stops under that will give you a nice fill ratio to keep the shadows from blocking up.
I would also put another bed sheet in front of the mirror so it doesn't case you fits with direct light bouncing around. I'm surprised it wasn't mentioned already other then in your question. Yea, it can cause havoc if you let any direct light hit it.
If you want to keep it simple, use the sunlight from the windows which would be a nice effect by itself, and either use the reflector to create fill, or use the SB800 behind a sheet for diffused fill that you can dial up or down as you prefer.
A light meter would be a good help, but you can also dial in the the lighting on the LCD panel. To do this, start with the 'constant' that you can't change. In this case, that would be the sunlight if you decided to use that. Get the exposure dialed in for the effect you want on the sunlight, then start adding in the fill from the SB800 until you get the effect you want there.
You can also open the blinds all the way, then cover the window with a bed sheet and use that as a large softbox. Then use a reflector to add in fill, or use the SB800 as a hard light source for accent, hair light, rim lighting, etc.
Lots of ways you can approach this. I hope this gives you some ideas to kick around.
If you want to be a bit more adventurous, here is a creative way to use that mirror to create some interesting lighting with only a single strobe.
You can tape patterns on the mirror, then hit it with direct light from the strobe, and it will create shadow patterns on your scene or subject. For example, I use a small 2'x3' acrylic mirror for that purpose. I tape random shapes of black paper to it, then hit it with a hot light or strobe and bounce the light onto my scene. The area without paper show up as light, and the areas with paper are shadow.
Here is an example of it in use, but with a clear mylar sheet instead of a mirror. Same principle, but lighted through the mylar, rather than bouncing off it. You are seeing the back side of the tape which is actually black from the front. But the same principle applies. Note the interesting effect it has on the lighting on the wall. Position your subject to take best advantage of it, and you'll get some nice drama in your lighting and final image.
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Here is the result of the above lighting. It could also have been done with a strobe, but I used hot lights since one was already setup. While a more advanced technique, its as simple as it gets as far as equipment: 1 light, sheet mylar, tape.
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