If I withdraw mid-Mains (after Day 1 or Day 2), does it count as an attempt?

Yes — but only because the Prelims you sat earlier already counted. Mid-Mains withdrawal does NOT add a SECOND attempt. The attempt was 'used' the moment you appeared in Prelims. Practical impact: you've lost a chance at AIR this year but your attempt counter ticks up by exactly 1, not 2.