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You beat me to it, so all I can do is Sign Off on it:):)
The Wiz is really the only answer, for her other movies she got decent reviews and box office.

Not quite - while the photo shoot and images were likely inspired by "Mahogany," I believe that the Cleopatra subplot is referring to Diana Ross' next film "
The Wiz (1978)
," in which she played the lead role of Dorothy. Ms Ross came under a lot of flack for playing a part that should have gone to someone closer to the character's age - which, incidentally, was Deena Jones' chief objection to her manager / husband insistence that she play someone in a movie that for most of the time would have been 16 yrs old

However, it is in the same point in the timeline
And a "Theme from Cleopatra" would be aired as part of TOTP in an alternate April 1976

No. Not really. However, the huge blow ups of Deena in Curtis' office and bedroom and the "When I First Saw You" montage with Deena in various fashions including her "Cleopatra" costume and make-up were inspired from the 1975 film, "Mahogany".
This should come as no surprise as "Dreamgirls" is considered a fictionalized version of the rise of Diana Ross & The Supremes.