> Critically, problem solving questions should have ways to progressively increase and decrease difficulty/complexity, so every candidate "gets a win" and no candidate "dunks the ball".

Absolutely agree. When I interview, I start with a simple problem and add complexity as they go. Can they write X? Can they combine it with Y? Do they understand how Z is related?

Same. I'm never doing a fail/pass type interview. Instead I try to assess where the candidate is on the beginner/intermediate/expert axis and match that with the expectations of the role I'm interviewing for.