To the bolded... no, because LFR is the most popular one, per the dev statements I've seen on the matter. The exact ratio that transition from N to H is not really known, and whenever it's the majority or not I doubt is very important personally. It happens because Heroic is also fairly popular, just less so than Normal and especially LFR.
And that's a pretty damn arbitrary distinction RE difficulty. Different tuning is still a different difficulty, and sometimes mechanics have to be changed to accommodate smaller raid sizes, albeit it's been so long I even set foot in Ulduar that I couldn't cite specifics. But judging by this weird standard of yours, LFR and Normal Denathrius are the same difficulty given that they have the same mechanics, just with varied tuning. That's not an opinion likely to be upheld by the majority, is all I'll say to that.