Hello again, I had no luck (though I also had little time to devote so my check may not be as thorough as I would like it to be) with weights, at this point, if anyone is in the mood to take a look, I would be in debt to whoever figures out what’s messing things up.
Another thought I had: earlier I had messed up the skin modifier (not on this model), and the mesh appeared face-down on the ground, in the T-pose, and if I remember correctly it didn’t animate, or if it did, it was these IK handlers in particular. There may be a chance that the IKs are in that same location for whatever reason, but I have no idea why they would do that, or why only they would do that, and not the entire skeleton.
You can get the FBX here (don’t worry, the link should remain active for a good 2-3 years or so)