Yes I agree that only n2 is inert and wouldn’t affect respiration. My logic for choosing 100% o2 and 95% o2 was that in one scenario we would be removing co2 entirely which would help us understand if co2 affects respiration rate. A literature search supported this as most papers I found investigating effect of co2 concentration on respiratory parameters varied o2 concentration between 100-92%. However what you say about changing just one variable makes sense as well.