When can two pronouns being compared in a sentence be of different types?

Solution 1:

Well, there simply is no rule about needing to matching pronouns/noun phrases in sentences using comparisons "more" or than". I would say the right way to think about the prescription is that you "should" use the proper case for the pronoun in the context of its own clause (or an imaginary clause that it is part of).

For example, consider this sentence: "In the enthusiasm of his passion he thought her more beautiful than she really was" (Consuelo).

There is no error here because the first pronoun takes its case according to its role in the surrounding clause (object—"he thought her") and the second pronoun takes its case according to its role in its own clause (subject—"she really was [beautiful]"—actually, it's not grammatical to explicitly add the word "beautiful" here, but that's the implied structure of the clause).

Similarly, people who prescribe using "he" in "She is taller than he" would as far as I know also prescribe using "he" in a sentence like "It is impossible for her to be taller than he".