The thing about safety items is there are no "must have" items. You never see divers with the same gear, some have more, some have less, some have just the most basic gear (especially those who rent) and then you have divers such as me who carry just about every safety device and backup scuba gear that can be found.
It's all about your risk tolerance. You may never need any of the stuff, or it could save your life. I know I am much more at ease in the water knowing I have the following
2 dive lights (in addition to the two video lights attached to my Gopro rig which can also be used as dive lights)
19cf pony bottle with it's own regulator
In addition to the above 2nd regulator I have a BCD mounted octopus so I have 3 regs
2 cutting tools (although I often leave one in my dive bag when diving known sites where there probably won't be any monofilament fishing line)
primary dive computer, backup dive computer and analog spg to backup my 2 air integrated computers
safety sausage
SMB and reel
air horn on my BCD inflator hose
whistle
signal mirror
In my dive bag
extra mask and extra mask and fin straps
zip ties
assorted o-rings and clips
computer batteries and o rings and computer battery cover removal tool
more batteries for dive lights and camera than I will probably ever need
extra 1st and 2nd stage regulators
tools to swap out regulators and hoses