Yes, the slim and non-slim variants are identical. Since the duration-only variants only show the icons and overlaid text, not of the off-to-the-side text of the left and right variants, there's nothing left to slim down.
Yes, they should show your static buffs. The window variant only determines the display of each item, the game determines what effects are considered maintained spells. Most until-cancelled buffs are.
Stack count is the increment count, yes. From what I understand the Carnage ability should have two icons. The actual ability with increments should show as one icon, and doesn't have a duration. The proc it can trigger has a duration but no increments and should be a separate detrimental applied to the mob, and I believe it shows up in the maintained window just like eg. poison procs do when they trigger on a mob. I had to work my mystic through the new signature line and get her ready for raiding, my next project is to bring up either my berserker or wizard to get a class that can have stacked effects. That'll let me directly see what's happening.
"Hey! I'm not illiterate. I had a book once. ...could've used some ketchup though..."