Which preposition should one choose when having two adjectives that require different prepositions and different verb forms in a comparative?

The Elements of Style by William Strunk Jr. and E. B. White says, “Some words require a particular preposition in certain idiomatic uses. When such words are joined in a compound construction, all the appropriate prepositions must be included …” (p. 44 in the 2000 illustrated edition). It gives an example, “His speech was marked by disagreement and scorn for his opponent’s position,” and the corrected form, “His speech was marked by disagreement with and scorn for his opponent’s position.”

By this rule, your example would be correctly written as “The ruling class is incapable of more than unwilling to pursue the public interest.” That sounds right to me too.