How to see which objects need repair

As already stated, buy the maps, once you have the maps you can set a waypoint that will disappear when you are close to the mask however it usually is close enough to give you an idea of the level the mask is located.

Secondly look for a dead body and a note, the mask is left by a serial killer at the scene of the crime, this will let you know that you are definitely in the right location and the mask should be nearby.

Finally listen for the audio cue, as you wander around in the blue circle you should hear a low note repeated that will start quietly and increase in volume as you get closer to the mask.

Also worth noting is that the mask isn't always out in the open, look around under tables, in boxes or underwater. It can also be in an inaccessible location, in this case you can shoot it.

Without giving away too many spoilers, there shouldn't be any restrictions or punishments for taking down a fortress while the difficulty is "hard". Longinus quests have nothing to do with fortresses. The story missions would eventually weaken the fortress but there's nothing stopping you from doing them early.

The main benefit to taking over the fortresses is that outposts in the area won't be subjected to random take-over attempts.