What is the flaw in treating every following sentence as istiʾnāf bayānī?

Supplementary Practice 9: How Small Wording Changes Alter Rhetorical Meaning

• Istiʾnāf always requires coordinating wāw
• The implied question and its answer have not been established
• Istiʾnāf occurs only in nominal clauses
• The second clause must always substitute word-for-word for the first