I may be able to help on this particular point.
My understanding is that the expansion is to allow, in the retroactive scheme, the addition of conspiracy to commit murder to the list, for the purposes of the DNA data banking regime and the warrant scheme. Expanding this beyond the persons involved in committing or attempting to commit the very serious offence of murder would also extend it to those who have the propensity to assist in that kind of crime. Even though they may not be leaving.... They could, I suppose, if they did this by leaving DNA on an envelope; if it's conspiracy, it's possible that you could have DNA evidence linking them directly. But it would also mean the police would be able to collect their DNA profile for data banking purposes in the future, allowing designated offences to be connected to them, should they commit those offences in the future.