im currently training to become a funeral director intern, studying mortuary science. haven't done a wound like this hands on, but it is covered in textbooks and i've studied it. fyi obviously everything differs heavily from place to place, person to person, and the state of the body, especially in violent messy murders.

so, how did they make her head presentable? well: via the neck, an artery and jugular are slightly pulled out, exposed and cut. a cannula is inserted into the artery attaching it to the embalming machine, which is filled with all the fluid. the jugular gets a small hole, and angular forceps are inserted into the vein to hold it open. as the injecting begins pressure builds up and pushes the congealed blood out through the jugular, replacing it with embalming fluid. then everything would be placed back inside and sewn up.

now in biancas case, based on the picture it looks like the left side of her neck was the focus. so they could have used her right artery and jugular, or just fished the ends of her left down far enough to be able to inject. beyond that, there's facial arterys near the jawline etc that can be used too, so they would have found a way. eye caps will have been placed under her eyelids (usually with a sticky gel called stay-cream) to keep them closed. her jaw would be clamped shut with a needle injector. it shoots a wire with a spike into the bone behind the gums, above the top teeth and below the bottom teeth, the mouth is closed and the wires are twisted together.

they would have used a piece of dowel or pvc in the spinal column to give everything structure again, use large cross sutures on the back of the neck to stabilise it onto the body, and then fully suture the head back on. and if there was any sinkage/loss of volume it would have been filled in with something like paper, styrofoam, etc. everything would be covered and rebuilt with mortuary wax and cosmetics.

apologies if any of that is confusing or misspelled, it's late where i am and i should probably be asleep lol.

if in the end they decided it was still too disturbing for her family to see directly, she would have been covered with a casket veil that drapes over the body so she is still visible but details/wounds are hard to see.