OK, it took a couple days, but I solved it. In order to bypass the FBX issues caused by Blender, I had to build an animation splitter plug-in.
I didn’t even realize I had this skill…
This gave me access to just the animation in the left arm, and loaded it as a core animation, which brought the arm out of the T pose for the default jump… This might even look good once I replace that animation, which apparently makes arms flap in the breeze.
But for now, I have defeated the beast! Wow, that was painful… I hope Roblox has some better tools coming for skinned meshparts and animations. I have tons of ideas, but no access to those discussions.