A candidate's current stage tells you where the application sits now. Stage history tells you where it has been. Pipeline conversion uses history: count the candidates who reached the later stage, then divide by the candidates who reached the starting stage within the same requisition population. Someone who advanced to interview still belongs in the screened cohort.
Keep the cohort unchanged between numerator and denominator. A rejected candidate may still have completed screening, while a withdrawn applicant remains an application from the recorded source. Source of hire counts completed hires rather than every application a channel supplied. Those counts answer different questions. A source with many applicants can still have no hires in the observed records. Small cohorts also limit the recommendation you can make: report the counts with the rate, and avoid treating one successful hire as a reliable forecast of the next campaign.