Oh no... I hope everyone is ok. No neck pain or anything I hope..
*I am not an insurance adjuster or non destructive technician*
In your 5th picture, it looks like the seam has let go, as it was designed. As well in the back, the crumple zone also looks like it has begun to fold as it has been designed to.
Newer cars with unibody designs are designed with front, rear and side crumple zones, surrounding a reinforced safety "cage" if you will. The crumple zones are typically single or laminated single sheet steel, designed to crumple on impact as the force of the impact is absorbed by making the metal bend and contort as needed. Less force reaches the safety cell and in turn, less force is brought onto the driver. It is intentionally weak for a purpose. For example, the indents in the trunk on pictures 5 and 7 are specifically designed to bend and fold a certain way in a rear end collision.
Another way to look at it is the front of the micra. It has a pitiful front bumper for a reason, the bending and crushing of that weak metal and the ac condenser, rad, etc is designed to dissipate the energy of the collision before it reaches the occupants. The weird damage we see in modern crashes is all by careful design, even our headrests in the micra are specifically sized to prevent whiplash. Even the air gap between the front bumper cover and the actual bumper itself is there for pedestrian safety, along with the slope of the hood in an attempt to minimize injury to the pedestrian. The engine mounts are designed with specific grade bolts and will break away in an impact to allow the engine to drop and not be pushed through the firewall.
The problem with crumple zones is that they are extremely difficult to repair once damaged. Steel once bent is no longer as strong as when it was cast and if a second impact occurs will not hold the same characteristics as it did originally. Certain items like seatbelts, the webbing can become stretched in an impact and if the pretensioners activate and lock, the seatbelts will have to be replaced. (As a side note, the position of the belts when the pretensioner activates can tell an investigator if the seatbelt was worn at the time of impact. If the seatbelt was not on, it will usually be tight against the door panel.) If you had any carseats in the vehicle at the time, they should also be replaced as they are only designed for one impact.
All of these items can be quite expensive, as is labor at a proper body shop that can repair the floor pan.
Long story short, the micra has performed as it was designed, and is most likely a write-off, but thankfully you were rear ended, so the fault isn't on you. (Also, Im not a police officer, but that seems to be the way it goes.)
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View my fuel log 2015 Micra S automatic: 7.9 L/100 km ... 35.7 mpg (Imp) ... 12.6 km/L ... 29.7 mpg (US) ...