Citadel physical dimensions: 5 wards, each ~45 km x ~5 km x 330 m. That gives a single ward around 74.25 cubic km, or 7.425 x 10^10 cubic meters for total Citadel volume of around 371 cubic km (3.71 x 10^11 cubic meters). The Citadel's mass is given as 7.11 billion metric tons, or 7.11 x 10^12 kg. Total density of the Citadel is ~19.16 kg/m^3, which is pretty low (fiberglass insulation is around 20 kg/m^3). We can assume lots of empty space making up each ward.
The Citadel is rather pliable. Even if we assume super-compressed and specialized fictional alloys, its internal joints and struts aren't going to handle the pressures put on the overall structure. That is to say, it's going to warp almost immediately once it hits atmo, and the heavy heating its internal air pockets experience along with general ablation means it's going to rip and blow itself apart into smaller pieces very, very quickly.
The cinematics in Mass Effect 3 after the destroy ending show the lower-right ward separating at what I estimate to be around 2.5 km/s. Considering the Citadel is in stationary orbit over London (51° N latitude) and is pointed generally towards Pluto/Charon (17° inclination in 2186), that gives an angle of around 34°, so we can assume a vertical space-to-surface motion of around 1.3 km/s. We have to note the stationary orbit, as it means initial v is zero.
Impact models don't really do non-round objects with fiberglass densities, so we're going to have to take some liberties here. Note that I'm an astrophysicist and not a materials scientist or structural engineer, so accept that there's going to be some leeway in what I expect.
The ward is going to just start falling while maintaining a north-to-south general motion of about 2.1 km/s. Where the ward starts impacting the atmosphere depends entirely on how high above it Earth it actually was. I'm going to make things easy for myself, and assume it was just starting to kiss the atmosphere.
It's going to start experiencing intense bending forces between 30-and-40 km up, at which point it will start to split apart very rapidly. Dynamic pressures are around 60 kPa at 30 km of altitude, rapidly increasing to 150 or so by 20-25 km. By that point, heated pockets should have ripped apart most of the structure and torn the ward into pieces. Your negative drag acceleration is going to be between 1-2.5 g that isn't evenly distributed across the surface, so even long intact structs are going to bend like crazy.
As the structure breaks up, pieces with greater average density will fall faster as their terminal v is greater than the structure's as a whole. Intense heating will ablate these pieces into molten blobs of metal or vaporize as a polymer (I was going to text Sash and ask her the chemistry part of this, but she's teaching a class right now). This leads to the ward pieces leaving a long chain of airbursts and impact craters along its length and N-S direction of motion, probably a few hundred km long. Those airbursts are the biggest issue, as they'll flatten anything in their vicinity when they finally dump most of their energy into the surrounding air. Think of a long chain of thousands of kiloton explosions with a scattering of megaton explosions thrown in. As pieces impact at different times, pressure bursts will accelerate other parts into a wider band that spreads the damage outward.
The heat of the deorbiting is surprisingly small. A basic E=(1/2)m*v^2 gives about 4.4 x 10^18 J, which is a little over a gigaton. Not all of that is going to be thermal, so expect 200-400 megatons in equivalent thermal energy. The atmosphere along this path is going to be hellish, but overall it's not a doomsday event. Most of the ward materials are going to be vaporized and distributed throughout the atmosphere, so climatological effects are going to be worse as those materials will likely reflect sunlight or block it completely, causing a global cooling event. People on Earth are going to suffer some intense breathing problems for a few months until the particulate matter settles.
Fortunately, with the Citadel over London, the direction of motion is south over the Atlantic ocean. There will be some major tsunamis, but most of the material damage falls into the ocean and doesn't leave too much damage over land. Earth is going to be changed drastically, but this disaster is nothing compared to the Reapers' destruction across the planet in the months prior.
Again, I'm an astrophysicist. If i made a mistake somewhere: it happens. This was all just for fun while I held office hours where no one ever shows up.
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