1.50.2 Fix for worker mode now released.
https://github.com/gritsenko/c3_spine_plugin/releases/tag/1.50.2
For the bounding box are you doing the UpdateBBoxes ACE before calling the expressions? Also, see the example project.
From the README.md:
SpineBBoxCenterX, SpineBBoxCenterY expressions give the average of the named slot/bbox polygon points. Useful for attaching C3 Sprite object/collision box to the center of the Bounding Box.
The UpdateBBoxes updates the bounding box to the current point in the animation. It should be done just once per tick per instance before the SpineBBoxCenterX/Y expressions are used.
Also check the dev console, do you see any errors or warnings?