I activated mine at t1 above departures... very quick and easy.

You absolutely can use it to enter then exit within 6hrs. I flew in on ek at 0100 T3 and out 0330 T1 QR (before the tensions of course). Only issue is it doesn't let you through automatically it'll send you to the officer who just asked why such a short stay, then waved me through (no stamp)

(non resident - uk passport)