Realness is definitely a problem. I think by now a mayor reason could just be "trolling paranoid people", as they're generally a willing target and have no real defense against this sort of thing.

Snowden has a reputation, so he's unlikely to be fake. The Independent would also ruin their rep by publishing fakes - but might still. It's not unlikely Snowden/The Intercept are helped by people with various motivations.

German court couldn't have been able to determine the slides real, therefore must treat them false for the purpose of proof. Doesn't mean they're actually fake, it's just not certain enough that they're real.