This is just something I figured out after way too long, and I wanted to share it in case anyone else was dealing with a similar issue.
I was dealing with a very frustrating issue with the custom skeletal mesh that I was importing into uefn so I could use it as a replacement custom npc guard skin. Every time I was using the IK retargeter it would put my target skeletal mesh (the one I imported) way up into the air even though in blender and elsewhere the base/root of my mesh sat at 0, 0, 0. I had no idea why it kept shoving my imported mesh/rig or whatever way up into the air.
I tried out the character in game anyways to see what would happen. I initially thought that the npc was invisible, as I only saw its gun moving around when the npc spawned. I later figured out that the npc was actually way up in the air. It was roughly 10,000 centimeters in the up.
The problem was that my skeletal mesh that I created and imported from blender was meauring its units in meters instead of centimeters like uefn or Fortnite does. That was throwing my skeletal mesh off 100x from where it was supposed to be. The uefn and blender units were not equal to each other, and the fbx file carried that over into uefn.
I fixed it by changing the blender units to centimeters instead of meters. I put the scaling parameter to 0.01 instead of 1. I then increased the size of my armature/skeleton and the mesh itself by 100x I believe. I exported that new file and when in the IK retargeter the Fortnite mannequin IK rig, and my custom skeletal mesh IK rig then lined up almost perfectly. The imported mesh was no longer majorly offset. Scaling.