Lauren would be back on the booze and not risking her life to find Lucy's killer" to take the scent off her! It's why Peter, Denise, Ian and Abi wouldn't make sense either.
It has to be someone cold and calculated enough to stay calm through this all, someone in a mess not making any sense or someone we see little of. Which is why the undertaker, Charlie, Aleks, Ben all make sense. And the only one who is acting a little crazy is Ian and he's too much of a wimp and loved Lucy. But if they want it to be emotional I would have thought Ben.
Ben would make sense as he has form. The undertaker could be a secret silas type. And Charlie and Aleks seem like they are capable if Lucy had something on him.