"What's your source for the 'Harriet' controversy being 'because of her GREAT COMET commentary in particular and her reactionary, tone-deaf form of social activism in general?' Is that purely your own desire, or is there a basis for this reading?"
No source or basis other than that what I consider to be her ignorant and toxic GREAT COMET commentary, which got a lot of publicity and therefore, I think, may well have contributed to the reaction against her casting as Harriet Tubman, whereas that casting might not have received any objections if she hadn't felt it necessary to comment on THE GREAT COMET.
I don't believe you have actually made a statement on whether you personally consider Erivo's GREAT COMET commentary to be in any way sensible, right, and justifiable or, on the contrary, highly ignorant and tremendously damaging. If the former, we really don't need to discuss this matter any further, because that would indicate we have diametrically opposed perspectives on that horrendous situation.