Alright! So I tried about everything, and with your suggestions I was able to pinpoint the exact thing that was causing the issue.
First of all, I was using the auto retargeter and not creating retarget assets, but exporting the animations directly. So one of the things I tried was creating the ik rig and ik retarget assets and exploring them.
The auto system wasn’t considering breast and facial bones as we suspected to be one of the issues, but it was doing something odd: instead of using the clavicle bones as start for the arm chains, it was starting from the shoulder bone, and creating “clavicle” chains with only the clavicle bones in it. this was causing the arm position issue, and some of the upper back arching.
Similarly, the head chain was composed of the single head bone, and an additional neck chain was created, containing the only neck bone.
I merged the neck and head bones into a single head chain, deleted the clavicle chains and added the clavicle bones as start for the arm chains.
this fixed about 95% of the noticeable issues as you can see from just the base pose:
after that it was just a matter of fine tuning and manually positioning the bones to match the source skeleton pose, and I can’t notice anything wrong.
I hope this helps anyone else having similar issues. Thanks for all your help, folks!