Correct HAMO - checkin opens 3 hours prior to pushback and indeed closes minus forty.
If you have an agent with any brains, your seat has been preallocated, and failing airport intervention you will get it. More likely to get it if you check in in a timely maner.
A service one will not generally find offerd through Fright Centre, Student flights and other "here is your ticket - have a nice lifez" outlets.
Best regards
EWL