Can a spreadsheet replace an ATS?
It can support a very small, low-volume process, but it becomes fragile when multiple users, communication, interviews, permissions, automation, retention, and reporting matter.
What should be tracked for every active candidate?
At minimum track the role, stage, owner, last activity, next action, next-action due date, and a factual disposition when the process closes.
Should the spreadsheet contain interview notes?
Keep notes job-related, factual, and access-controlled. A dedicated ATS is usually safer and more usable for detailed interview feedback and scorecards.