My experience of overnighting at MAN following a connecting flight from GLA is;-
1. I could not enter API online - I used the ICC. By the time I had checked in the API had disappeared
Spare yourself the trouble - just do the API at check in.
2. When on an overnight connection your luggage cannot be through checked. You can do the night before check in at desk 23 between 7 and 9 pm.