So I had an idea… the UE4 Skeleton has a LOT of wonderful assets associated with it on the marketplace, and I thought, “Hey, the retarget manager doesn’t say that the Target skeleton HAS to be adjusted to look like the UE4 Source skeleton…”
So I went, exported the Mannequin to my desktop, uploaded it into Mixamo, chose the T-Pose animation, downloaded it, Imported into UE4 with the setting set as an animation, and utilizing the skeleton of the existing Mannequin (so it doesn’t make its own).
Opened the animation, went to the retarget manager, saved the t-pose as the retarget pose, and saved the skeleton. I made sure to use the retarget manager on BOTH skeleton and I set up the Humanoid Rig. I went to the browser to the UE4 Mannequin skeleton, right-clicked, retarget to another skeleton, chose the Mixamo Skeleton. It works PERFECTLY.
Thing is, what it animates is the delta from the base pose. I was always under the impression that the Target required to match the Source, but as it turns out: it doesn’t matter!
TLDR: I fixed it. You have to read to have the instructions how.
P.s. - - Enjoy the t-pose animation file!