If by joint mangling you mean the collapsing shoulders and creasing thighs on V4, that is due to magnet/JCM interaction and has nothing to do with the rotation controller. One very common cause is you have applied a pose that un-hid some of the normally hidden JCM dials, and Carrara does not like that. At the very top of V4 in the morph dials there is a “deformer strength” -> set it to -1.
Otherwise you are just running into the limitations of the Poser rigging system: A3’s thighs go flat and weird, V4 has really bizzare armpits… that’s just how it is.
Quaternions solve exactly one problem in Carrara: If you are trying to make something spin and you are having trouble smoothly going between -180 and +180 degrees, then it can help. Set it at the very top level, or better yet, a completely separate group. I’m sorry to contradict Holly, but I would recommend that you don’t ever change the figure itself.
Change Rotation Controller is part of Posing Pack 3, which contrary to my earlier statements is NOT removed from the store: http://www.daz3d.com/software/plugins-utilities/posing-pack-3 DAZ. decided that it was their way or no way, so now I have to re-evaluate my strategy. I was hoping for something a little more substantial than “8.5 late this summer, 9 to follow soon-ish”, but there you go.
Edit: In that thread, Holly was specifically talking about rotations beyond a full circle - not individual joints. That is quite right. It is NOT gimbal lock, it is interpolation: the tweeners don’t know how to interpolate between -180 and +180 and get a smooth motion.