if its true that Dennis fakes his own death then cool... but obviously sharon would have to be in on it. The motive? Perhaps that old chestnut Jack Daltons name crops up again courtesy of either Johnny Allen or Chrissy (maybe Den Snr had told Chrissy about Dennis killing Jack when they were "happily married"). If the evidence (what evidence i have no idea) was concrete enough to send young rickman down then I suppose the choice of pretending to be dead or spending life in jail for killing a well known ganglord you can see how easy that choice would be to make.