The crimping of the cartridge is incredibly interesting, to me. I think
@Hexe has talked before about the possibility some propellant had been removed from the round in order to reduce the sound of the shot. AIUI, to do this you’d first need to open up the cartridge, then afterwards crimp it back together.
Or, was the crimping done simply to misdirect and confuse?
And as for the gun itself,
if you were acting out some violent sexual fantasy, why go to the bother / take the risk of fashioning / sourcing a working firearm when almost anyone would freeze with the barrel of a replica pointed at their head?
IMO, I think whoever had the expertise to modify the gun and possibly the round - or, whoever took the risk of sourcing a modified gun and round - went to Jill Dando’s doorstep that morning with the very clear intention of ending her life.