It gets overly complicated in my view when one tries to use the on camera type flashes to shoot these types of shots. That is why so many people have problems with using them.
Basically, if you can just treat your flash as a constant light source with a known power, then you'll find it much easier to do this sort of thing. So, first check to see if you can make the flash always fire full power all the time or 1/2 power, etc. In other words you want it to be under your control, not the camera.
Now you need to establish the exposure of the subjects using the flash only. If you don't have a flash meter, then the use a gray card, or a black/gray/white target will allow you zero in on the flashes power and get you the correct fstop for your camera (you'll be using the histogram for this).
Once you have the fstop, that is what you leave the camera set to. Now you meter the background to determine the shutter speed needed for the fstop that is already set. You are now ready to shoot and both subject and background will be properly exposed.
Now for the reflectors. You can use them either to bounce light from the sky into the scene to alter the ratio of the light, or you can set them to reflect some of the flashes light back into the subjects. Again, this is all about getting a proper lighting ratio.
Here is a link to a brief outline of the target based metering technique: