think

WB I'd hafta see context. The implication of a third party "her" confuses the issue If not I might say

Not only John but Mary herself would think her the protagonist

Mary would think herself the protagonist just as John does

John thinks Mary the protagonist while so does she, herself

John and she herself would think Mary the protagonist

It is correct.

I can see why your former teacher would prefer the form "John and Mary would think of her as the protagonist", but your form is actually better as well as shorter.

McTag is right. This form... I was worried you would think me a fool; they think him a liar, people think him odd is perfectly fine and acceptable. maybe a little old fashioned and slightly formal but not at all incorrect.

I think of Jane Austen when I read sentences like that.

Excellent answers. Thank you.

Quite welcome WB

Alternate context: The two friends thinking it over together, in the eternal conflict between Harriet and her husband, John and Mary would think her the protagonist.