The phase thread stuff is here https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_GenPurposeLoopingMacros.html%23flu_udf_sec_THREAD_SUB_THREADS (scroll up a bit).
I can't comment on the undocumented macros. However, you may want to check the way C counts as real uvel = cell->V may be looking at the vvel! C count 0, 1, 2 etc so directions are x=0, y=1 and z=2.
The Eulerian multiphase model is for more highly packed particles/bubbles/droplets. DPM is good to about 10-14% volume fraction. The latter is computationally cheaper, and works in a different way.