[Dr. J Chakma]

I would like to mention that upekkha is not a cetasika. There are 6 pakinnaka (vitakka, vicara, viriya, piti, chanda and adhimokkha). Out of 25 sobhana cetasika 19 always arise in every kusala citta (hence no question of them arising singly or only some of them). Remaining 6 sobhana cetasika are samma vaca, samma kammanta, samma ajiva, karuna, mudita and panna. There is no upekkha cetasika. Upekkha is actually a state of mind and need to be cultivated and it is not neutral in true sense. I think we can say that when a citta have only 7 universal cetasika with or without one or more of particulars (pakinnaka) can be said to be cultivating upekkha.
Regarding Phabhassara citta of arahants I have to think and look for reference. I think Lal can clarify this point.
With METTA to ALL beings