Did you close up the mini styro nucs for 24 hrs in a cool location? they should of stayed put after that.I will be following that routine as I try to populate my styrofoam mating nucs this year. Last year was a disaster with getting enough bees to remain and I lost several newly emerged queens. This year I intend to get some brood in the mini frames first and then do the 24 hour qe trick to get enough bees for the nucs.
This is what I do also. I have done splits literally side by side and had great success.Same. Only move my splits a few feet in most cases. I make sure to find my queen and shake a lot of extra bees into the split. Nurse bees won't fly back, foragers will. You may not notice a lot of activity for a few days in the split because of this, but they will be just fine as long as you get enough nurse bees.
I do the same thing with my queen castles and mating nucs. Shake bees in from brood frames and notice very little drift back.