I am trying to create a mannequin model where the abdomen box bends and twists when I move the thorax or pelvis. The stretched side (left from viewer) looks good but the squashed side looks weird.
You have a weird setup. You have a hip bone connceted to a pelvis bone which goes up to the ripcage ( to big ) and is moving the lower body (usuall upper)… so any subdivision is not going to work as expected… also there are some remnants of a former rig ? The complete ripcage has a spine3 bone but also vertex groups spine2, spine3, spine.001, spine.002, spine.003, spine.004, spine.006 and pelvis.L, pelvis.R.
I guess you should re-search some skinning , armature, rigging tutorials to make youself a bit more familiar with this, because this setup is… just weird.
Ahh… now i see… well there are many different kinds of posible rigs and i was your pelvis bone expection to be something like root bone or something because pevis is more the bones and hip more the body part but both are associated to the same area… (at least for me ) . Also for forward or/and reverse kinematics. So the difficulty is to “see” for what it is meant for and the remnants i spoke of are the different (unsused) vertex groups… which should be (and are) deletable.
Another very very big difficulty is the workflow in rigging… not only someone has to know a lot about it to build the rig in this way so that does any “special” feature someoen wants but also any alterations is causing an avalanche of finetuning… for example re-weight-painting… so some automatic weighting is nice to have when doing the inital skining… but some alteration need someone to know what to do and be very very concentrated about it.
So my suggestion about subdividing the bones may work nicely without explicite weight painting when doing so from the the start but you have already some weight an this and indeed i also struggle wehnit comes to changes in the armature
Another thing i discovered: you have some weight for the pelvis bone butn ot for the hip bone… i’m not sure if this is meant this way…