Are there any benefits for placing a sign on your building

An abandoned building will eventually be reacquired if its problems are resolved. You can see why the building was abandoned by clicking on it to bring up information that building.

In many instances this will be a reason like the building having no water or power, there not being enough jobs within commute distance, pollution or backed up sewage, or the building being damaged by fire.

Abandoned buildings will have a negative effect on the land value of nearby buildings, and have an increased chance of catching fire. Keeping abandoned buildings around over the long term will drive down land value for the surrounding area and it is usually recommended that once the building goes 'dark' (visibly darker than the surrounding buildings - like the one in your screenshot) that it has begun to decay and will soon reach a point that it is better to bulldoze that building to prevent it from causing damage to the value of the surrounding buildings.

When you activate the bulldozer tool, all abandoned buildings of any zone type will begin flashing with an orange symbol with a line through it to make them easier to locate.

No - I wouldn't be worried about overwriting buildings. From the beta testing I did I didn't notice any negative effects from destroying properties while placing new structures.

You are right that overlapping a structure over an RCI zone will not dezone that area - meaning that any buildings destroyed will free up the remainder of the space to allow new RCI buildings to be built in the smaller space that is left.

Destroying the RCI building would have removed the home or job for whichever Sims live or work there, but the employment model in SimCity is that agents simply travel around until they find a workplace requiring the type of agent they are (ie: high wealth).

In the event that any unhappiness is generated by the placement of the new structure, this would be counteracted by the happiness generated from placing the new structure.

For example, you will generate happiness in the region by placing schools, parks, police or fire stations in a residential area, placing casinos or other commercial buildings in commercial sectors, or placing large industrial buildings (including power plants) in industrial zones.