Thanks for updating us.
It is very difficult to say exactly what the problem is without seeing the character, but in general check the issues below.
First make sure the T-Pose has been setup correctly. It can often appear right at first glance, but often bones can be rotated incorrectly. So pay particular attention to the hip, torso area. You can go into Characterize, turn off HIK, make adjustments, and then check how it looks with the Calibration options in the Animation Player (under the viewport).
Regarding spine and neck bones, you don't need to assign them all. You only need assign the bones based in your characters bone rig, but always start from the bottom up in the Characterization window.
Peter
Forum Administrator
www.reallusion.com