Hopefully, I will be EUA’d into FC on all segments (or doesn't Mexico count? There are
so many loop holes!).

yes, for upgrade eligibility Mexico counts the same as regular domestic flights. I dont know about CUN, but I am almost always upgraded to MEX, which was one reason I tried going via EWR.
Thanks SBM.
RNE - maybe you will share with us your pictures from the beaches CUN......... =)