"While" followed by simple past/past continuous

As an English teacher, I believe it's more correct to say 'While she was waiting' if we're talking about school grammar.

According to Oxford Practical English Usage, the simple past tense can refer to an extended/longer action in 'simultaneous long actions'. i.e. one long + one long action e.g. John cooked supper while I watched TV. (or ...was cooking... was watching...)

But for 'background' action/situations, i.e. one short + one long action, the past continuous is used for the longer 'background' event in contrast. e.g. While they were playing cards, somebody broke into the house. e.g. As I was walking down the street I saw Joe driving a Porsche.

The example you quoted obviously belongs to the latter case. Hence, "While she was waiting [LONG] for Walter to pad away, Molly noticed [SHORT] how close her face was to Cole's..."

Use of the simple past does not preclude an extended action / state, or there wouldn't be a simple past tense of say 'wait'.

He waited while I combed my hair.

He stayed there for several months.

The past continuous is used when there is a more punctive occurrence during that process:

He saw a mouse as he was waiting while I was combing my hair.

While he was staying there for several months, he was discovered by an MI7 agent.