you can get behind them by going through them. i have practiced with folks whose training involved staying on the line and taking the center as well as folks who always getting off line. i found that folks who trained to stay on the line didn't have problem getting off line; that's just another option for them. however, folks who always trained with getting off the line, had a hard time stay on the line when required, i.e. put them on a log bridge exercise.
I can see your point there but I would expect that those who trained with getting off line that you mention were probably taught that it was to evade or escape.
Both should be done as completely separate disciplines in my mind.
Without excess terminology I would demonstrate the two and in so doing show the difference.
One I would show as enter, finish. Or enter,cut, finish. As one motion.
The other (irimi) I would show as enter, allow others cut to complete, finish.
Two different skills with totally different effects on uke.