Didn't think it would turn into a Frontier bash thread but oh well.
I can tell you that a majority of gate agents will not remove non revs for revenue passengers once the non revs are boarded. As it says on the boarding pass, "doors close 15 minutes prior to departure" Seen it happen in multiple stations. Most of the agents are looking out for the non revs. ( I will agree not all of them may share that sentiment, like any other airline your experience will vary depending on the individual that day) I've been pulled off a flight last minute multiple times on multiple airlines at departure time due to last minute passengers. Its all part of the game, sometimes you win, sometimes you lose.
I think thats the bottom line we all need to remember when non reving/jumpseating regardless of the airline.