keeping skin tone consistent: use the same ratio of ambient-to-flash, and a set, fixed WB when shooting. Keep lighting and camera settings (f/stop/shutter/ISO) the same, and when importing the raw files, use the same LR preset. Your shots should be pretty consistent if you go about it the right way.
if you run in auto white balance,and allow the camera exposure to vary much, you can get some difference in the original file, and that means 'correcting' a lot. The more parameters you keep the same, the easier it is to keep the final shots close.When shooting flash+ ambient, the color temp of the flash's light can vary, with monolights being, allegedly, worse than box-and-cable systems...so, flash at Full or half power may be different in color temp than flash at 1/8 power,etc. Also--flash + ambient light...varies with the modifier's color...I have 3 umbrella and a softbox from1986..VERY yellow light from them...I do not use them any more.
Flash-to-daylight ratio--- flash power level,and camera exposure settings...are they close from shot to shot?