How much notice is reasonable when the handover is complex?
Whatever the contract says, plus a written handover that outlives you: runbooks, access list, the three things that break at 3am. Extending the notice period rarely helps; writing things down always does.
Any way to opt out of whiteboard interviews without seeming difficult?
Offer a substitute rather than a refusal: a take-home you have already done, a walkthrough of public work, or pair debugging on their codebase. Companies that decline all three have told you something useful about how they work.
How do you estimate a task with unknown parts?
Timebox the unknown separately. Estimate the known work normally, then add an explicit investigation slot with a decision point at the end. Merging the two produces a number that is wrong in a way nobody can inspect.
What is the most common way people get this wrong?
Doing it once and never verifying. The setup is the visible part, so it gets attention, and the check that would catch a silent failure never gets written.