Alright, the checklist in my Guide is complete. The Canister was the last object I needed for the achievement! Thanks man As for needing to perform the different variations, I don't think you do, as long as you do them once with a Gadget User and once with a Power User. So only twice, no need for the extra fancy variations with Deathstroke like someone mentioned in the Forum's Thread.
Also regarding power characters and gadget characters and their interactions with objects. For such things as the bat signal I don't know if you have to use both on objects like those. However I've noticed subtle differences such as Atlantis both side walls power characters break the glass and gadget characters spin the valve. On arkham and Jokers asylum the flower can be thrown by power characters while gadget characters step on the vine below it. I've also noticed subtle variations like these with the oil truck and neon sign in gotham city the airplane in the museum in metropolis and the far right side pillar with the gargoyle statue atop on waynes manor. I wonder if those could be what's missing.
Here's what I can figure so far from what I've seen of your list and the things I've done. Things I saw missing were on Atlantis the pipe across the top of the map a gadget character can slide from one side to the other and the canister in the museum. As far as the canister goes I'm not sure if the grenades you mentioned are it because on the far left there is a case with an rpg that can be unlocked after damage and once used the canister follows. Also I don't know if it counts but I figured it was worth noting also in the museum on the far right is an exhibit you can kick an opponent through.