Germans were executed for carrying sawback bayonets because...any excuse is a good one I guess...it broke no rules, but it looked nasty and troops kill a lot of people during assaults, irregardless of status. The brits hadn't issued a sawback in decades at that point, but the practice had been quite widespread, typically either 1-2 per squad and/or exclusive to sappers and engineers throughout Europe. Remember these were the days of specialized carbines/musketoons for artillery, sappers, MPs, ect. No clue as to their usefulness, but the idea was that it gave you a saw you could use, without issuing another item. They were commonly called "pioneer bayonets" and similar in the local tongue.
The practice fell out after WWI because it had become too common to execute legal combatants carrying them.
It's somewhat of a more cool-headed version of the vein of executing flame-thrower operators, though those were retained for their obvious usefulness until PC bullshit, willful ignorance and limp-wristed distaste saw to their decline.
more germane to the subject;
@OP; i'd seriously consider if you are in possession of stolen property. I'd not show that item around if I were you and in fact as it is a crappy knife and a worse bayonet; I'd toss it. Not sell (stolen, remember?) toss it and chock it up to life experience.
As to the rest; it is a blade; therefor it should be sharpened and kept clean and free of rust. This applies to all the bladed instruments in your Qs; including, yes; shovels. And yes, sigh, it DOES make them work better, obviously.
Bayonets should be of an all-metal (shiny) appearance on the blade; this parkerizing (or whatever finish it is) bullshit is asinine and reduces the primary value of the bayonet; intimidation. Bayonets have two roles; 70% scaring the shit out of the enemy; thus making a show of flashing steel when you fix. A pointed rifle says; "I am going to kill you" but that fails to resonate in the hind brain you want to get at. A fixed bayonet says; "I am going to come over THERE and kill you, by exploring your insides with a cold hunk of sharp metal" That does the job; keys right into the natural fear of a mammal towards teeth and claws.
25% is for firming up the morale and fighting spirit of your own guys and it works too; look at history for examples.
The remaining 5% is for a little bit of Close-Quarter work.
Beyond that; frankly the discussion is more trouble in the CF, especially on this forum than it is worth with LOAC vs Laws of War and the limited understanding of both that pervades all levels of the CF.