Yesterday I was mucking around with the kids who were in the pool and I was on our balcony upstairs. They wanted me to lower their afternoon tea in a basket to them. I had to lean right over and my chest was squashed and as I reached out to swing the basket closer to them, my right armpit was slightly abraded on the balcony rail (I am right handed).
I can easily visualise gbc heaving poor Allison over the rail of the bridge, her tooth being chipped in the process, his chest being bruised from her weight "pulling" him forcefully against the bridge railing and then as he is dangling her over the edge, her jumper slips up her body to over her head and he leans to the right because he is right handed (do not know if this is the case...) and the last swing on her body (trying to heave ho her out as far as possible) before he lets go, causes a "hickey" type sub dermal bleeding rash at the junction of his arm and armpit. Just a theory moo.
MS I had this exact thought this morning! It would also explain why Allison's hands were tangled up in the jumper, if she had been thrown down head first in the way you described.