Closing a line is has been always allowed(when it is not longer allowed it will be on Quest rules) however if a boss get stuck, does not fight back or not going after the empire anymore each are bugs and you caused them replay will not be allowed anymore.
If I use 2 well-know mechanics of the game in a way that buys me as much time as I want in late game (some kind of variation on the Michael stall, but a LOT more efficient), is it considered an exploit ?
Buying infinite time is not a valid game, on the valid version for quest is not so easy to do it as it was on the previous version.
Knowing that I'm not using any trick/glitch that I know of.
If you are not sure you can send me a PM asking for it. Though if you are not sure is because is probably a bug.