will I need to re-sight the shotgun any time the barrel is removed?
It shouldn't make much if any difference in POI (point of impact), as long as you are consistent with the amount of torque placed on the magazine cap when you reinstall the barrel. I usually snug it up, then go one more 'click' on the detent - always by hand, no tools.
the placement of the sights now doesn't make sense to me
Ghost rings work BECAUSE the rear sight is near your eye, and your eye (if you let it) will ignore the rear aperture and automatically center up the front sight. Once you learn not to deliberately look at the rear sight, but look through it and focus on the front sight, the GRs will get to be very fast for you to use.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.