You wrote: “There was no black-haired baby, Gendry is not her baby, none of those theories made any political sense whatsoever.” What about the possible clues left in by GRRM about Gendry’s mom having golden hair, and Cersei’s habit of using disguise? Would they be mere red herrings?

Those would not be possible clues. Gendry’s mother having fair hair is a clue that Robert only has black hair genes, Cersei using a disguise the one time is a complete non-sequitur. 

None of these theories hold up to any level of scrutiny. As Queen of Westeros, Cersei cannot conceal a pregnancy to term, nor could she easily escape the castle to raise a secret child in Flea Bottom, and her childbirths are incredibly scrutinized events. 

But even were it possible, it goes against basic motivation. Cersei didn’t want to have Robert’s child succeed to the Iron Throne. That being the case, she has no damn reason to go through a pregnancy when she could very easily end it, which she tells us she did in AGOT.